Add html-aam spec guidance on CSS accessibility treatment and mappings

[rahimabdi]

Although the general purpose of CSS is to separate presentation from content/functionality, there has been increased discussion lately in the ARIA WG around CSS's impact on HTML accessibility:

• The content-visibility CSS property across browsers #572
• Acknowledge text-transform impacts a11y tree #554
• Issue: CSS Highlights Not Clearly Documented in AAM #550

I spoke with @scottaohara today and we propose that as a first step towards documenting CSS accessibility behavior, it would be valuable to add a new section in HTML-AAM explaining CSS's influence on the accessibility tree and assistive technology user experience. This new spec guidance could also include an initial list of CSS features that impact accessibility and their documented accessibility API mappings, e.g., the following:

• Properties
  • content
  • visibility
  • display
  • font (font-style, font-weight, font-family)
  • list-style-type
  • position
  • order (for flexbox/grid)
  • text-decoration
  • text-transform
  • pointer-events
• Pseudo-classes
  • :checked
• Pseudo-elements
  • ::before
  • ::after
  • ::highlight
  • ::marker
  • ::selection
  • ::grammar-error
  • ::placeholder