[css-ui-4] influence of outline-width on auto style outlines

[frivoal]

CSS-UI claims that it's up to user agents to determine whether and how outline-width influences the rendering of outlines with outline-style auto.

However, this is problematic, for the similar reasons as discussed about color in #7761:

outline: auto sets outline-style to auto and outline-width to medium, and medium means 3px.

The default UA sheet in the HTML spec has :focus-visible { outline: auto; }, and so, in browsers that want to take outline-width into account, it effectively calls for the default rendering of focus outlines to be native outlines somehow tweaked to be 3px wide. That seems unfortunate.

I think we should either:

• remove the claim that outline-width is allowed to influence the rendering of auto style outlines.
• similarly to what we did in [css-ui] Tweaking outline-style: auto colors. #7761, allow outline-width to influence the rendering of auto style outlines, but to make that work sensibly, add an auto initial value to outline-width which computes to medium when outline-style is anything but auto, but lets auto style outlines render at their natural size unless the author has explicitly called for something else.

[Loirooriol]

Does any browser let outline-width affect the size auto outlines?

[frivoal]

As far as I know, neither Safari, Firefox, nor Chrome do it.
As far as my tests could confirm, the only print formatter which does is BFO Publisher.

@faceless2 is this something that matters to you?

[faceless2]

If it looks like it might matter to us, it's almost certainly a bug 😁 I thought we simply treated auto as solid - no impact on width at all.

[Loirooriol]

If you treat auto as solid, then it's normal for it to be affected by outline-width. Same for Servo.

I was referring to the native outlines.

[frivoal]

Agenda+ to propose removing the claim that outline-width is allowed to influence the rendering of auto style outlines

[css-meeting-bot]

The CSS Working Group just discussed [css-ui-4] influence of outline-width on auto style outlines, and agreed to the following:

• RESOLVED: Remove the claim that outline-width influences the rendering of auto style outlines

The full IRC log of that discussion

<dael> astearns: This was florian
<dael> TabAtkins: Look straightforward
<dael> astearns: Prop: Remove the claim that outline-width influences the rendering
<dael> astearns: Obj?
<dael> astearns: Prop: Remove the claim that outline-width influences the rendering of auto-style outlines
<dael> RESOLVED: Remove the claim that outline-width influences the rendering of auto style outlines

[Loirooriol]

I disagree with the test. We (Servo) do not intend to treat auto different than solid for now, and this is allowed by the spec ("User agents may treat auto as solid"). solid obeys outline-width, so we are failing the test.

Please change the test to also accept a different expectation with div { outline: solid 100px} div+div { outline: solid 0px } or such.

[frivoal]

@Loirooriol , good point. Made a follow up PR to adjust the test, review appreciated: web-platform-tests/wpt#43010