[css-display] More precise box transformations

[Loirooriol]

CSS Display has some handwaving about display types belonging to elements or boxes. Mostly it's clear what it means, but I think that Box Type Transformations should be more precise, and differentiate between elements, element-generated boxes, anonymous boxes, and text runs.

I think it should say something like this:

• Definition: blockifying a principal box blockifies the generating element.
• Definition: inlinifying a principal box inlinifies the generating element.
• Definition: blockifying or inlinifying a text run does nothing.
• Definition: blockifying an element sets its outer display type to block and, if its inner display type is layout-internal, it is set to flow.
• Definition: inlinifying an element sets its outer display type to inline unless it was layout-internal. If the inner display type is flow, the box recursively inlinifies its in-flow children.
• Note: blockifiying or inlinifying an element without display types has no effect.
• Note: blockifiying or inlinifying an element affects the computed value of display.
• Note: blockifications and inlinifications happen before the creation of anonymous boxes to fix the box tree.
• Assert: CSS specifications MUST prevent text runs from remaining as direct children of a parent box which blockifies its contents, e.g. by wrapping the text runs inside appropriate anonymous boxes.
• Assert: CSS specifications MUST prevent anonymous inline-level boxes from being generated as direct direct children of a parent box which has blockified its contents, e.g. by generating anonymous block-level boxes instead.
• Assert: CSS specifications MUST prevent anonymous block-level boxes from being generated as direct direct children of a parent box which has inlinified its contents, e.g. by generating anonymous inline-level boxes instead.
• Assert: CSS specifications MUST prevent text runs, boxes and elements from being simultaneously blockified and inlinified.
• Assert: CSS specifications MUST prevent non-principal element-generated boxes from being blockified or inlinified.

[tabatkins]

We're not entirely sure what are all the problems you're trying to solve with this issue, but there seem to be at least two:

• Make sure that we reflect style computation from boxes to elements for all properties correctly when an element generates multiple boxes.
• Record the guidelines for authoring box generation rules; this probably goes in an appendix since it's aimed only at spec writers.

Was there something else that needs fixing?

[Loirooriol]

No, I think this was all.

[fantasai]

Agenda+ for WG verification:

• box ↔ element style value correspondance in b2d484c
• box construction spec guidance in 533ac0a

[fantasai]

@Loirooriol Please let us know if these edits are satisfactory!

[Loirooriol]

Much better, thanks! But I'm still not convinced that boxes have display types. Boxes may have characteristics like level or formatting context, but probably only elements and tree-abiding pseudo-elements (i.e., things to which display applies) should have display types. So I would prefer to define box blockifications and inlinifications in terms of element blockifications and inlinifications (which are not currently defined). Do you think it's worth?

[c-smile]

We refused many times to introduce separate outer display and inner display / flow properties. But each time we are bumping into problems of that too vague display thing.

Like flexbox inside table-cell still prohibited by current CSS state of affairs.

It really should be something like this:

display: table-cell;  /*outer requirement */
flow:flexbox;          /* content layout style - flexbox */

and

display: list-item;  /*outer requirement */
flow:flexbox;         /* content layout style - flexbox */

in the same way:

display: table-cell;  /*outer requirement */
flow:grid;          /* content layout style - grid */

Otherwise we will keep inventing that wheel forever.