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<h1>CSS Table Module Level 3</h1>
<pre class='metadata'>
Shortname: css-tables
Level: 3
Status: ED
Work Status: Exploring
Group: csswg
ED: https://drafts.csswg.org/css-tables-3/
TR: https://www.w3.org/TR/css-tables-3/
Previous Version: https://www.w3.org/TR/2016/WD-css-tables-3-20161025/
Previous Version: https://www.w3.org/TR/CSS2/tables.html
Repository: w3c/csswg-drafts
Warning: not ready
Editor: Francois Remy, Microsoft
Editor: Greg Whitworth, Microsoft
Former editor: Bert Bos, W3C
Former editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Former editor: Markus Mielke, Microsoft
Former editor: Saloni Mira Rai, Microsoft
Abstract: This CSS module defines a two-dimensional grid-based layout system, optimized for tabular data rendering. In the table layout model, each display node is assigned to an intersection between a set of consecutive rows and a set of consecutive columns, themselves generated from the table structure and sized according to their content.
Ignored Terms: block-level box
</pre>
<pre class=link-defaults>
spec:css22; type:property; text:min-width
spec:css22; type:property; text:max-width
spec:css22; type:property; text:top
spec:css22; type:property; text:right
spec:css22; type:property; text:bottom
spec:css22; type:property; text:background
spec:css22; type:property; text:border-style
spec:css22; type:property; text:border-width
spec:css22; type:property; text:overflow
spec:css22; type:property; text:white-space
spec:css22; type:property; text:display
</pre>
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<h2 id="intro">Introduction</h2>
<em>This section is not normative</em>
Many types of information (ex: weather readings collected over the past year)
are best visually represented in a two-axis grid
where rows represent one item of the list
(ex: a date, and the various weather properties measured during that day),
and where columns represent the successive values of an items property
(ex: the temperatures measured over the past year).
Sometimes, to make the representation easier to understand,
some cells of the grid are used to represent a description or summary of their parent row/column,
instead of actual data.
This happens more frequently for
the cells found on the first row and/or column (called headers)
or the cells found on the last row and/or column (called footers).
This kind of tabular data representation is usually known as tables.
Tables layout can be abused to render other grid-like representations like calendars or timelines,
though authors should prefer other layout modes
when the information being represented does not make sense as a data table.
The rendering of tables in HTML has been defined for a long time in the HTML specification.
However, its interactions with features defined in CSS remained for a long time undefined.
The goal of this specification is to define
the expected behavior of user agents supporting both HTML tables and CSS.
Please be aware that some behaviors defined in this document
will not be the most logical or useful way of solving the problem they aim to solve,
but such behaviors are often the result of compatibility requirements and not a deliberate choice
of the editors of this specification.
Authors wishing to use more complex layouts
are encouraged to rely on more modern CSS modules such as CSS Grids.
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<h2 id="content-model">Content Model</h2>
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="table-structure">Table Structure</h3>
The CSS table model is based on the HTML4 table model,
in which the structure of a table closely parallels the visual layout of the table.
In this model, a table consists of an optional caption and any number of rows of cells.
In addition, adjacent rows and columns may be grouped structurally and
this grouping can be reflected in presentation (e.g., a border may be drawn around a group of rows).
The table model is said to be "row primary" since
authors specify rows, not columns, explicitly in the document language.
Columns are derived once all the rows have been specified:
the first cell of the first row belongs to the first column
and as many other columns as spanning requires (and it creates them if needed),
and the following cells of that row each belong to the next available column
and as many other columns as spanning requires (creating those if needed);
the cells of the following rows each belong to the next available column for that row (taking rowspan into account)
and as many other columns as spanning requires (creating those if needed).
<span class="hint">(see [[#dimensioning-the-row-column-grid]])</span>.
To summarize, an instance of the table model consists of:
<ul class="compact">
<li>Its <a href="#table-root-element">table-root</a> containing:
<ul>
<li>Zero, one or more <a href="#table-row">table rows</a>, optionally in <a href="#table-row-grouping-element">row groups</a>,
<ul><li>Each of them contaning one or more <a href="#table-cell">table cells</a></li></ul>
<li>Optionally: one or more <a href="#table-column">table columns</a>,
optionally in <a href="#table-column-group">column groups</a>
<li>Optionally: one or more <a href="#table-caption">table caption</a>.
</ul>
</ul>
<style>
ul.compact { margin-top: -1em !important; list-style-type: disc; }
ul.compact li { margin-top: 0 !important; margin-bottom: 0 !important; list-style-type: disc; }
</style>
<figure>
<img src="images/table-structure.png" width=493 />
<figcaption>Two representations of the structure of a table (tree vs layout)</figcaption>
</figure>
The CSS model does not require that the document language include elements that correspond to each of these components.
For document languages (such as XML applications) that do not have pre-defined table elements,
authors must map document language elements to table elements.
This is done with the 'display' property.
The following 'display' values assign table formatting rules to an arbitrary element:
<dl id="display-types">
<dt><dfn>table</dfn> (equivalent to HTML: <table>)
<dd>Specifies that an element defines a block-level table:
it is a rectangular block that participates in a block formatting context.
<dt><dfn>inline-table</dfn> (equivalent to HTML: <table>)
<dd>Specifies that an element defines an inline-level table:
it is a rectangular block that participates in an inline formatting context).
<dt><dfn>table-row</dfn> (equivalent to HTML: <tr>)
<dd>Specifies that an element is a row of cells.
<dt><dfn>table-row-group</dfn> (equivalent to HTML: <tbody>)
<dd>Specifies that an element groups some amount of rows.
<dt><dfn>table-header-group</dfn> (equivalent to HTML: <thead>)
<dd>Like table-row-group but, for layout purposes,
the first such row group is always displayed before all other rows and row groups.
<div class="advisement">
If a table owns multiple <code>display: table-header-group</code> boxes,
only the first is treated as a header;
the others are treated as if they had <code>display: table-row-group</code>.
</div>
<dt><dfn>table-footer-group</dfn> (equivalent to HTML: <tfoot>)
<dd>Like table-row-group but, for layout purposes,
the fist such row group is always displayed after all other rows and row groups.
<div class="advisement">
If a table owns multiple <code>display: table-footer-group</code> boxes,
only the first is treated as a footer;
the others are treated as if they had <code>display: table-row-group</code>.
</div>
<dt><dfn>table-column</dfn> (equivalent to HTML: <col>)
<dd>Specifies that an element describes a column of cells.
<dt><dfn>table-column-group</dfn> (equivalent to HTML: <colgroup>)
<dd>Specifies that an element groups one or more columns.
<dt><dfn>table-cell</dfn> (equivalent to HTML: <td> or <th>)
<dd>Specifies that an element represents a table cell.
<dt><dfn>table-caption</dfn> (equivalent to HTML: <caption>)
<dd>Specifies a caption for the table.
Table captions are positioned between the table margins and its borders.
</dl>
<p class="advisement">
Authors should not assign a display type from the previous list to replaced elements (eg: input fields or images).
When the 'display' property of a replaced element computes to one of these values,
it is handled instead as though the author had declared
<code>inline</code> (for <a>inline-table</a>, <a>table-cell</a>, <a>table-column</a>) or
<code>block</code> (for all other values).
Whitespace collapsing and box generation must happen around those replaced elements
like if they never had any other display type.
<br/>
</p>
ISSUE(508): This is a breaking change from css 2.1 but matches implementations
<h4 id="terminology">Terminology</h2>
In addition to the table structure display types,
the following wording is also being used in this spec:
<dl>
<dt><dfn id="table-wrapper-box">table-wrapper</dfn> box
<dd>
An anonymous box <a href="#fixup-algorithm">generated around table-root boxes</a> to account for any space occupied by each <a>table-caption</a> it owns.
<dt><dfn id="table-root-element">table-root</dfn> box or element
<dd>
A <a>table</a> or <a>inline-table</a> box.
<dt><dfn id="table-non-root-element&q
6E5C
uot;>table-non-root</dfn> box or element
<dd>
A <a>proper table child</a>, or a <a>table-cell</a> box.
<dt><dfn id="table-row-grouping-element">table-row-grouping</dfn> box or element
<dd>
A
<a>table-row-group</a>,
<a>table-header-group</a>, or
<a>table-footer-group</a> box.
<dt><dfn id="table-track">table-track</dfn> box or element
<dd>
A
<a>table-row</a>, or
<a>table-column</a> box.
<dt><dfn id="table-track-grouping-element">table-track-grouping</dfn> box or element
<dd>
A
<a>table-row-grouping</a>, or
<a>table-column-group</a> box.
<dt><dfn id="proper-table-child-element">proper table child</dfn> box or element
<dd>
A
<a>table-track-grouping</a>,
<a>table-track</a>, or
<a>table-caption</a> box.
<dt><dfn id="proper-table-row-parent-element">proper table-row parent</dfn> box or element
<dd>
A <a>table-root</a> or a <a>table-row-grouping</a> box.
<dt><dfn id="table-internal-element">table-internal</dfn> box or element
<dd>
A <a>table-cell</a>, <a>table-track</a> or <a>table-track-grouping</a> box.
<dt><dfn id="tabular-container">tabular container</dfn>
<dd>
A <a>table-row</a> or <a>proper table-row parent</a> box.
<dt><dfn id="consecutive-boxes">consecutive</dfn> boxes
<dd>
Two sibling boxes are consecutive
if they have no intervening siblings
other than, optionally, an anonymous inline containing only white spaces.
A sequence of sibling boxes is consecutive
if each box in the sequence is consecutive to the one before it in the sequence.
<dt><dfn id="table-grid">table grid</dfn>
<dd>
A matrix
containing as many <dfn id="row">rows</dfn> and <dfn id="column">columns</dfn>
as needed to describe the position of all the <a>table-rows</a> and <a>table-cells</a> of a <a>table-root</a>,
as determined by the <a href="#dimensioning-the-row-column-grid">grid-dimensioning algorithm</a>.
Each row of the grid might correspond to a <a>table-row</a>, and each column to a <a>table-column</a>.
<dt><dfn id="slot">slot</dfn> of the table grid.
<dd>
A <a>slot</a> <code>(r,c)</code> is an available space created
by the intersection of a row <code>r</code> and a column <code>c</code> in the <a>table grid</a>.
Each slot of the table grid is covered by at least one <a>table-cell</a> (<a href="#missing-cells-fixup">some of them anonymous</a>), and at most two.
Each table-cell of a table-root covers at least one slot.
Table-cells that cover more than one slot do so densely,
meaning the set of slots it covers can always be described as a set of four strictly-positive integers <code>(rowMin, colMin, rowSpan, colSpan)</code>
such that a slot <code>(r,c)</code> is covered by the table-cell
if and only if <code>r</code> lies in the interval between <code>rowMin</code> (included) and <code>rowMin+rowSpan</code> (not included),
and <code>c</code> lies in the interval between <code>colMin</code> (included) and <code>colMin+colSpan</code> (not included);
Such table-cell is said to <dfn>originate</dfn> from row <code>rowMin</code> and column <code>colMin</code> (and, by extension, the corresponding table-tracks).
Such table-cell is said to <dfn>span</dfn> all rows <code>r</code> and columns <code>c</code> matching the above condition (and, by extension, the corresponding table-tracks).
</dl>
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<h3 id="fixup">Fixup</h3>
Document languages other than HTML may not contain all the elements in the CSS 2.1 table model.
In these cases, the "missing" elements must be assumed in order for the table model to work.
Any <a>table-internal</a> element will automatically generate necessary anonymous table objects around itself, if necessary.
Any descendant of a <a>table-root</a> that is not table-internal
must have a set of ancestors in the table consisting of
at least three nested objects corresponding to
a <a>table</a>/<a>inline-table</a>,
a <a>table-row</a>, and
a <a>table-cell</a>.
Missing boxes cause the generation of <a href="#fixup-boxes">anonymous boxes</a> according to the following rules:
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="fixup-algorithm">Fixup Algorithm</h4>
For the purposes of these rules, out-of-flow elements are represented as inline elements of zero width and height.
Their containing blocks are chosen accordingly.
The following steps are performed in three stages:
<ol>
<li><b>Remove irrelevant boxes:</b><br/>
The following boxes are discarded as if they were <code>display:none</code>:
<ol>
<li>Children of a <a>table-column</a>.
<li>Children of a <a>table-column-group</a> which are not a <a>table-column</a>.
<li>Anonymous inline boxes which contain only white space and
are between two immediate siblings each of which is a <a>table-non-root</a> box.
<li>Anonymous inline boxes which meet all of the following criteria:
<ul>
<li>they contain only white space
<li>they are the first and/or last child of a <a>tabular container</a>
<li>whose immediate sibling, if any, is a <a>table-non-root</a> box
</ul>
</ol>
</li>
<li><b>Generate missing child wrappers:</b>
<ol>
<li>An anonymous <a>table-row</a> box must be generated
around each sequence of consecutive children of a <a>table-root</a> box
which are not <a>proper table child</a> boxes.
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/2/">!!Testcase</a>
<li>An anonymous <a>table-row</a> box must be generated
around each sequence of consecutive children of a <a>table-row-grouping</a> box
which are not <a>table-row</a> boxes.
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/3/">!Testcase</a>
<li>An anonymous <a>table-cell</a> box must be generated
around each sequence of consecutive children of a <a>table-row</a> box
which are not <a>table-cell</a> boxes.
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/4/">!Testcase</a>
</ol>
</li>
<li><b>Generate missing parents:</b>
<ol>
<li>An anonymous <a>table-row</a> box must be generated
around each sequence of consecutive <a>table-cell</a> boxes
whose parent is not a <a>table-row</a>.
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/5/">Testcase</a>
<li>An anonymous <a>table</a> or <a>inline-table</a> box must be generated
around each sequence of consecutive <a>proper table child</a> boxes
which are misparented.
If the box's parent is an inline box,
then an <a>inline-table</a> box must be generated;
otherwise it must be a <a>table</a> box.
<ul>
<li>A <a>table-row</a> is misparented
if its parent is neither a <a>table-row-grouping</a> nor a <a>table-root</a> box.
<li>A <a>table-column</a> box is misparented
if its parent is neither a <a>table-column-group</a> box nor a <a>table-root</a> box.
<li>A <a>table-row-grouping</a>, <a>table-column-group</a>, or <a>table-caption</a> box is misparented
if its parent is not a <a>table-root</a> box.
</ul>
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/7/">Testcase</a>
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/8/">Testcase</a>
<a class="hint" href="https://jsfiddle.net/hj2w7wwa/9/">!Testcase</a>
<li>An anonymous <a>table-wrapper</a> box must be generated around each <a>table-root</a>.
Its display type is <code>inline-block</code> for <a>inline-table</a> boxes and block for <a>table</a> boxes.
The table wrapper box establishes a block formatting context.
The table-root box (not the table-wrapper box) is used when doing baseline vertical alignment for an <a>inline-table</a>.
The width of the table-wrapper box is the border-edge width of the table box inside it.
Percentages which would depend on the 'width' and 'height' on the table-wrapper box's size are relative to the table-wrapper box's containing block instead, not the table-wrapper box itself.
</ol>
</li>
</ol>
<div class="note">
Please note that some layout modes such as flexbox and grid <a href="http://www.w3.org/TR/css-display-3/#blockify">override the display type</a> of their children.
These transformations happen before the table fixup.
</div>
<div class="note">
Please note that the 'float' and 'position' properties sometimes <a href="http://www.w3.org/TR/CSS22/visuren.html#dis-pos-flo">affect the computed value</a> of 'display'.
When those properties are used on what should have been table internal boxes, they switch to <code>block</code> instead.
This transformation happen before the table fixup.
</div>
<div class="note">
We have modified the text of this section from CSS 2.2 to make it easier to read.
If you find any mistakes due to these changes please file an issue
</div>
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="fixup-boxes">Characteristics of fixup boxes</h4>
Beside their display type, the anonymous boxes created for fixup purposes
do not receive any specific or default styling,
except where otherwise mentioned by this specification.
<div class="note">
This means in particular that
their computed background is “transparent”,
their computed padding is “0px”,
their computed border-style is “none”.
It is also woth reminding that an <a>anonymous box</a> inherits property values through the box tree.
</div>
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="fixup-examples">Examples</h4>
<div class="example">
<xmp class="lang-markup">
<div class="row">
<div class="cell">George</div>
<div class="cell">4287</div>
<div class="cell">1998</div>
</div>
</xmp>
Here is the associated styles:
<pre class="lang-css">
.row { display: table-row }
.cell { display: table-cell }
</pre>
After fixup, this will produce layout boxes as though this was the initial HTML:
<xmp class="lang-markup">
<table>
<tr>
<td>George</td>
<td>4287</td>
<td>1998</td>
</tr>
</table>
</xmp>
</div>
<div class="example">
In this example, three <a>table-cell</a> anonymous boxes are assumed to contain the text in the rows. The text inside
of the divs with a <code>display: table-row</code> are encapsulated in anonymous inline boxes, as explained in
<a href="https://www.w3.org/TR/CSS21/visuren.html#anonymous" target="_blank">visual formatting model</a>:
<xmp class="lang-markup">
<div class="inline-table">
<div class="row">This is the top row.</div>
<div class="row">This is the middle row.</div>
<div class="row">This is the bottom row.</div>
</div>
</xmp>
<pre class="lang-css">
.inline-table { display: inline-table; }
.row { display: table-row; }
</pre>
This will produce layout boxes as though this was the initial HTML:
<xmp class="lang-markup">
<table>
<tr>
<td>This is the top row.</td>
</tr>
<tr>
<td>This is the middle row.</td>
</tr>
<tr>
<td>This is the bottom row.</td>
</tr>
</table>
</xmp>
</div>
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<h2 id="layout">Layout</h2>
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="layout-principles">Core layout principles</h3>
Unlike other block-level boxes, tables do not fill their containing block by default.
When their 'width' computes to <code>auto</code>, they behave as if they had <code>fit-content</code> specified instead.
This is different from most block-level boxes, which behave as if they had <code>stretch</code> instead.
The <dfn id="min-content-width-of-a-table">min-content width of a table</dfn> is
the width required to fit all of its columns min-content widths and its undistributable spaces.
The <dfn id="max-content-width-of-a-table">max-content width of a table</dfn> is
the width required to fit all of its columns max-content widths and its undistributable spaces.
If the width assigned to a table is larger than its <a href="#min-content-width-of-a-table">min-content width</a>,
the <a href="#width-distribution">Available Width Distribution</a> algorithm
will adjust column widths in consequence.
This section overrides the general-purpose rules that apply to calculating widths described in other specifications.
In particular, if the margins of a table are set to <code>0</code> and the width to <code>auto</code>,
the table will not automatically size to fill its containing block.
However, once the used value of <code>width</code> for the table is found (using the algorithms given below)
then the other parts of those rules do apply.
Therefore, a table can be centered using left and right <code>auto</code> margins, for instance.
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="table-layout-algorithm">Table layout algorithm</h3>
To layout a table, user agents must apply the following actions:
<ol>
<li><b>Determine the number of rows/columns the table requires.</b><br/>
This is done by executing the steps described in [[#dimensioning-the-row-column-grid]].
<li><b>[A] If the row/column grid has at least one <a>slot</a>:</b>
<ol>
<li><b>Ensure each cell <a>slot</a> is occupied by at least one cell.</b><br/>
This is done by executing the steps described in [[#missing-cells-fixup]].
<li><b>Compute the minimum width of each column.</b><br/>
This is done by executing the steps described in [[#content-measure]].
<li><b>Compute the width of the table.</b><br/>
This is done by executing the steps described in [[#computing-the-table-width]].
<li><b>Distribute the width of the table among columns.</b><br/>
This is done by executing the steps described in [[#width-distribution-algorithm]].
<li><b>Compute the height of the table.</b><br/>
This is done by executing the steps described in [[#computing-the-table-height]].
<li><b>Distribute the height of the table among rows.</b><br/>
This is done by executing the steps described in [[#height-distribution-algorithm]].
</ol>
<b>[B] Else, if the table is empty:</b>
<ol>
<li><b>Compute the width of the table.</b><br/>
This is done by returning the largest value of <a href="#capmin">CAPMIN</a>
and the computed width of the table box (including borders and paddings)
if it is definite (use zero otherwise).
<li><b>Compute the height of the table.</b><br/>
This is done by returning the sum of all table-caption heights
(their width being set to the table width,
with margins taken into consideration appropriately)
and the computed height of the table box (including borders and paddings)
if it is definite (use zero otherwise).
</ol>
<li><b>Assign to each table-caption and table-cell their position and size.</b><br/>
This is done by running the steps of [[#bounding-box-assignment]].
</ol>
<figure>
<p class="note">
The following schema describes the algorithm in a different way,
to make it easier to understand.
</p>
<img src="images/CSS-Tables-Layout-Merged.svg" style="width: 100%" />
<figcaption>Overview of the table layout algorithm. Not normative.</figcaption>
</figure>
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="dimensioning-the-row-column-grid">Dimensioning the row/column grid</h3>
Like mentioned in the <a href="#table-structure">Table structure</a> section,
how many rows and columns a table has
can be determined from the table structure.
Both dimensioning the row/column <a>grid</a> and assigning table-cells their <nobr><a>slot</a>(s)</nobr> in that grid
do require running the HTML Algorithms for tables.
<h4 id="dimensioning-the-row-column-grid--step1">HTML Algorithm</h4>
<p class="note">
CSS Boxes that do not originate from an HTML table element equivalent to their display type
need to be converted to their HTML equivalent before we can apply this algorithm, see below.
There is no way to specify the <a>span</a> of a cell in css only in this level of the spec,
the use of an HTML td element is required to do so.
Apply the <a href="https://www.w3.org/TR/html5/tabular-data.html#forming-a-table">HTML5 Table Formatting algorithm</a>,
where boxes act like <a href="#display-types">the HTML element equivalent to their display type</a>,
and use the attributes of their originating element if and only if it is an HTML element of the same type
(otherwise, they act like if they didnt't have any attribute).
<div class="example">
<xmp class="lang-markup">
<ul class="table">
<li><b>One</b><i>1</i></li>
<li><b>Two</b><i>2</i></li>
<li><b>Three</b><i>3</i></li>
</ul>
<style>
ul.table { display: table; }
ul.table > li { display: table-row; }
ul.table > li > * { display: table-cell; }
</style>
</xmp>
produces the same row/column grid as
<xmp class="lang-markup">
<table><tbody>
<tr>
<td></td>
<td></td>
</tr>
<tr>
<td></td>
<td></td>
</tr>
<tr>
<td></td>
<td></td>
</tr>
</tbody></table>
</xmp>
</div>
<div class="example">
<pre class="lang-markup" style="margin-bottom:0;padding-bottom:0;">
<!-- built using dom api, as this would be fixed by the html parser -->
</pre>
<xmp class="lang-markup" style="margin-top:0;padding-top:0;">
<grid style="display: table">
<row style="display: table-row">
<th rowspan="2">1</th>
<colgroup style="display: table-cell" span="2" colspan="2">2</colgroup>
</row>
<tr>
<td>A</td>
<td>B</td>
<td>C</td>
</tr>
</grid>
</xmp>
produces the same row/column grid as
<xmp class="lang-markup">
<table>
<tr>
<th rowspan="2">1</th>
<td>2</td>
</tr>
<tr>
<td>A</td>
<td>B</td>
<td>C</td>
</tr>
</table>
</xmp>
Note how the second cell of the first row doesn't have ```colspan=2``` applied, because its originating element is not an HTML TD element.
<a class="hint" href="https://jsfiddle.net/eqrwaLyc/">Testcase</a>.
<a class="hint" href="https://jsfiddle.net/eqrwaLyc/1/">!!Testcase</a>.
<a class="hint" href="https://jsfiddle.net/sckxeLmh/2/">!Test case</a>.
<a class="hint" href="https://jsfiddle.net/sckxeLmh/4/">!!Test case</a>.
<a class="hint" href="https://jsfiddle.net/sckxeLmh/5/">!!Test case</a>.
</div>
<h4 id="dimensioning-the-row-column-grid--step2">Track merging</h4>
<p class="note">
The HTML Table Formatting algorithm sometimes generates more tracks than necessary to layout the table properly.
Those tracks have historically been ignored by user agents,
so the next step just gets rid of them entirely to avoid dealing with them as exceptions later in the spec.
We have tried to maintain the functionality with this change, but if you happen to find any issues
due to this change please file an issue.
Modify iteratively the obtained grid by merging consecutive tracks as follows:
As long as there exists an <a>eligible track</a> in the obtained row/column grid such that
there is no table-column/table-row box defining the said track explicitly, and
both the said track and the previous one are spanned by the exact same set of cells,
those two tracks must be merged into one single track for the purpose of computing the layout of the table.
Change the spanning of the cells contained in those tracks
accordingly so that no difference would be detectable in terms of slot assignment.
<span class="hint">(see <a href="#spanning-ghost-rows">spanning-ghost-rows test cases</a>)</span>
For tables <a>in auto mode</a>, every track is an <dfn>eligible track</dfn> for the purpose of the track-merging algorithm.
For tables <a>in fixed mode</a>, only rows are eligible to be merged that way; which means that every column is preserved.
Finally, assign to the <a>table-root</a> element its correct number of rows and columns (from its mapped element),
and to each <a>table-cell</a> element its accurate <a href="#slot">rowMin/colMin/rowSpan/colSpan</a> (from its mapped element).
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="missing-cells-fixup">Missing cells fixup</h3>
<p class=note>
The following section clarifies and extends the CSS 2.1 statement saying that
missing cells are rendered as if an anonymous table-cell box occupied their position in the grid
(a "missing cell" is a cell in the row/column grid that is not occupied by an element or pseudo-element).
Once the amount of columns in a table is known, any table-row box must be modified such that
it owns enough cells to fill all the columns of the table, when taking spanning into account.
New table-cell <a href="#fixup-boxes">anonymous boxes</a> must be appended to its rows content until this condition is met.
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="layout-modes">Table layout modes</h3>
This section covers the flags which modify the way tables are being laid out.
There are three major flags for table layout: 'table-layout', 'border-collapse', and 'caption-side'.
The 'border-collapse' flag has an optional 'border-spacing' parameter.
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="table-layout-property">The Table-Layout property</h4>
<pre class='propdef'>
Name: table-layout
Value: auto | fixed
Initial: auto
Applies To: table-root boxes.
Inherited: yes
</pre>
A table-root is said to be laid out <dfn>in fixed mode</dfn>
whenever the computed value of the 'table-layout' property is equal to <code>fixed</code>,
and the specified width of the table root is either
a <code><length-percentage></code>,
<code>min-content</code> or
<code>fit-content</code>.
When the specified width is not one of those values,
or if the computed value of the 'table-layout' property is <code>auto</code>,
then the table-root is said to be laid out <dfn>in auto mode</dfn>.
When a table-root is laid out <a>in fixed mode</a>,
the content of its table-cells is ignored for the purpose of width computation,
the aggregation algorithm for column sizing considers only table-cells belonging to the first row track,
such that layout only depends on the values explicitly specified for the table-columns or cells of the first row of the table;
columns with indefinite widths are attributed their fair share of the remaining space
after the columns with a definite width have been considered, or 0px if there is no remaining space
(see [[#computing-column-measures]]).
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="border-collapse-property">The Border-Collapse property</h4>
<pre class='propdef'>
Name: border-collapse
Value: separate | collapse
Initial: separate
Applies To: table-root boxes.
Inherited: yes
</pre>
When the 'border-collapse' property has <code>collapse</code> as its value,
the borders of adjacent cells are merged together such that each cell draws only half of the shared border.
As a result, some other properties like 'border-spacing' will not applied in this case (see [[#collapsed-style-overrides]]),
(see [[#border-collapsing]]).
A <a>table-root</a> element is said to be laid out <dfn>in collapsed-borders mode</dfn> in this case.
Otherwhise, the <a>table-root</a> element is said to be laid out <dfn>in separated-borders mode</dfn>.
<!--——————————————————————————————————————————————————————————————————————————-->
<h5 id="border-spacing-property">The Border-Spacing property</h5>
<pre class='propdef'>
Name: border-spacing
Value: <length>{1,2}
Initial: 0px 0px
Applies To: table-root boxes when 'border-collapse' is <code>separate</code>.
Inherited: yes
Computed Value: two absolute lengths
Media: visual
Animatable: yes
</pre>
The lengths specify the distance that separates adjoining cell borders <a>in separated-borders mode</a>,
and must not be strictly negative.
If one length is specified, it gives both the horizontal and vertical spacing.
If two are specified, the first gives the horizontal spacing and the second the vertical spacing.
See [[#computing-undistributable-space]] for details on how this affects the table layout.
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="caption-side-property">The Caption-Side property</h4>
<pre class='propdef'>
Name: caption-side
Value: top | bottom
Initial: top
Applies to: <code>table-caption</code> boxes
Inherited: yes
Media: visual
</pre>
This property specifies the position of the caption box with respect to the table box.
Values have the following meanings:
<dl>
<dt><dfn id="caption-side-top">top</dfn>
<dd>
Positions the caption box above the table box.
<dt><dfn id="caption-side-bottom">bottom</dfn>
<dd>
Positions the caption box below the table box.
</dl>
<div class="note">
CSS2 described a different width and horizontal alignment behavior.
That behavior was supposed to be introduced in CSS3
using the values <code>top-outside</code> and <code>bottom-outside</code>.
<a href="http://fantasai.inkedblade.net/style/discuss/captions/">#REF</a>
</div>
<div class="note">
Gecko also supports the "left" and "right" values, but currently this specification
is not attempting to define their implementation of said values.
</div>
<div classs="note">
Gecko has a bug when dealing with multiple captions.
<a class="hint" href="http://codepen.io/FremyCompany/pen/WrJxwP">!Testcase</a>
</div>
<div class="example">
To align caption content horizontally within the caption box, use the 'text-align' property.
In this example, the 'caption-side' property places captions below tables.
The caption will be as wide as the parent of the table, and caption text will be left-justified.
<pre>
caption {
caption-side: bottom;
width: auto;
text-align: left
}
</pre>
</div>
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="style-overrides">Style overrides</h3>
Some css properties behave differently inside css tables.
The following sections list the exceptions and their effects.
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="global-style-overrides">Overrides applying in all modes</h4>
The following rules apply to all tables, irrespective of the layout mode in use.
<ul>
<li>The computed values of properties 'position', 'float', 'margin'-*, 'top', 'right', 'bottom', and 'left' on the table box
are used on the table-wrapper box and not the table box;
the same holds true for the <a href="https://drafts.csswg.org/css-transforms/#grouping-property-values">properties whose use could force the used value of <object>'transform-style'</object> to <code>flat</code></a> and their shorthands/longhands relatives:
this list currently includes 'overflow', 'opacity', 'filter', 'clip', 'clip-path', 'isolation', 'mask'-*, 'mix-blend-mode', 'transform'-* and 'perspective'.
<br>
Where the table box values are not used on the table and/or table wrapper boxes,
the unset values are used instead for that box (inherit or initial, depending on the property).
<li>The 'overflow' property on the <a>table-root</a> and <a>table-wrapper</a> box, when its value is not either <code>visible</code> or <code>hidden</code>,
is ignored and treated as if its value was <code>visible</code>.
<li>All css properties of <a>table-column</a> and <a>table-column-group</a> boxes are ignored,
except when explicitly specified by this specification.
<li>The 'position' of <a>table-track</a> and <a>table-track-grouping</a> boxes is ignored
if its value is <code>relative</code>. User agents that support 'transform'
still need to support making these boxes containing blocks if required by spec.
<li>The 'margin', 'padding', 'overflow' and 'z-index' of <a>table-track</a> and <a>table-track-grouping</a boxes> are ignored.
<li>The 'margin' of <a>table-cell</a> boxes is ignored (as if it was set to 0px).
<li>The 'background' of <a>table-cell</a> boxes
are painted using a special background painting algorithm described in [[#drawing-cell-backgrounds]].
</ul>
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="collapsed-style-overrides">Overrides applying in collapsed-borders mode</h4>
When a table is laid out <a>in collapsed-borders mode</a>, the following rules apply:
<ul>
<li>The 'padding' of the <a>table-root</a> element is ignored (as if it was set to 0px).
<li>The 'border-spacing' of the <a>table-root</a> element is ignored (as if it was set to 0px).
<li>The 'border-radius' of both <a>table-root</a> and <a>table-non-root</a> boxes
is ignored (as it it was set to 0px).
<li>The values used for the layout and rendering of the borders of the <a>table-root</a> element
and the <a>table-cell</a> boxes it owns are
determined using a special conflict resolution algorithm described in [[#border-collapsing]].
</ul>
<!--——————————————————————————————————————————————————————————————————————————-->
<h3 id="border-collapsing">Border-collapsing</h3>
<div class="note">
This entire section is a proposal to make the rendering of collapsed borders sane.
As implementations diverge very visibly, it is expected to require more discussion than some other parts.
Since browsers handle this so differently, convergence cannot happen without reimplementation.
A major concern for this proposal was to support as many cases as possible, and yet
keep the effort required for a new implementation of tables as low as possible.
<b>Background:</b>
CSS+HTML allow unprecedented combinations of border modes for table junctions,
and it makes it difficult to support all cases properly;
in fact some combinations are not <a href="https://en.wikipedia.org/wiki/Well-posed_problem">well-posed problems</a>,
so no rendering algorithm could be optimal.
Because they grew from something simple (HTML) to something very complex (HTML+CSS),
the current table rendering models (backgrounds and borders) used by web browsers are insane
(in the sense they are buggy, not interoperable and not CSSish at all).
Many usual CSS assumptions are broken, and renderings diverge widely.
This proposal aims at fixing this situation.
</div>
ISSUE(604): border-collapsing breaking change from 2.1
<!--——————————————————————————————————————————————————————————————————————————-->
<h4 id="conflict-resolution-for-collapsed-borders">Conflict Resolution for Collapsed Borders</h4>
When they are laid out <a>in collapsed-borders mode</a>,
<a>table-root</a> and <a>table-cell</a> boxes sharing a border attempt to unify their borders
so that they render using the same style, width, and color (whenever this is possible).
This is accomplished by running the following algorithm.
<!--——————————————————————————————————————————————————————————————————————————-->
<h5 id="border-conflict-resolution-algorithm">Conflict Resolution Algorithm for Collapsed Borders</h5>
<div class="note">
For the purpose of this algorithm, “harmonizing” a set of borders means
applying the <a href="#border-style-harmonization-algorithm">“Harmonization Algorithm for Collapsed Borders”</a> on the given set of borders, and
set those borders' used values to the value resulting from the algorithm,
except for cells having a 'border-image-source' different from none:
those keep their initial values.
</div>
<b>For any <a>table-cell</a> element C° of a <a>table-root</a> element:</b>
<ul>
<li>Resolve conflicts with border-right:
<ul>
<li>Let C be an ordered set of <a>table-cell</a> element borders, sorted in Row/Column order of their parent cell;
initially, let C contain only C°’s border-right
<li>Add to the set C the border-left of all cells located after C°
sharing a section of their border-left with C°’s border-right
<li>Repeat the following two instructions, until no new cell is added to C:
<ul>
<li>For all newly-added cells C<sub>i</sub> having a rowspan greater than one,
add to the set C the border-right of all cells located before C<sub>i</sub>
sharing a section of their border-right with C<sub>i</sub>’s border-left
<li>For all newly-added cells C<sub>i</sub> having a rowspan greater than one,
add to the set C the border-left of all cells located after C<sub>i</sub>
sharing a section of their border-left with C<sub>i</sub>’s border-right
</ul>
<li>Harmonize the conflicting borders of C
</ul>
<li>Resolve conflicts with border-bottom:
<ul>
<li>Let C be an ordered set of <a>table-cell</a> element borders, sorted in Row/Column order of their parent cell;
initially, let C contain only C°’s border-bottom