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.