[css-display-3] Clarify display for ::first-line and ::first-letter

[karlcow]

Intro Comment

This issue was led by a discussion about a test on WPT.
http://wpt.live/css/css-display/display-first-line-001.html
https://github.com/web-platform-tests/wpt/blob/34dd07ed48/css/css-display/display-first-line-001.html

For example:

<style>
#t1::first-letter { float: left; display: flex; font-size: 30px }
</style>
<div id="t1">First letter is float and flex.</div>

and the test is

assert_equals(getFirstLetterDisplayFor("t1"), "block");

The test would gain to be improved by having

<div id="t1">First letter is specified as float and flex, but should return block for display</div>

So that led to searching in the spec to understand why it was supposed to be block, hence the issue:

The issue

In https://drafts.csswg.org/css-display/#placement

None of the properties in this module apply to the ::first-line or ::first-letter pseudo-elements.

but at the same time display defines its initial value as inline. So why would it be block?

Let's head for https://drafts.csswg.org/css-display/#transformations
In an example section, it says:

Absolute positioning or floating an element blockifies the box’s display type. [CSS2]

Pointing to the text just a bit above.

Some layout effects require blockification or inlinification of the box type, which sets the box’s computed outer display type to block or inline (respectively). (This has no effect on display types that generate no box at all, such as none or contents.)

So that's the answer which is part of the CSS display, so… some part of this module applies to ::first-line or ::first-letter.

So the sentence could be probably clarified

None of the properties in this module apply to the ::first-line or ::first-letter pseudo-elements.

Thanks @annevk and @anttijk for the discussions.

[Loirooriol]

I don't understand what exactly you want to clarify in css-display.

Also, I don't think the specs imply that the answer must be block. It could also be flex. display not applying doesn't necessarily prevent having a computed value, it could just prevent the first line from becoming a flex container.

Personally I would have used assert_in_array(..., ["block", "flex"]) or such.

[xiaochengh]

AFAIK using float in ::first-letter is a non-standard usage. Its only purpose is to support drop cap, and is superceded by initial-letter.

So I think the correct fix is to fix the test and assume display always computes to inline.