This will be the first PR in a larger effort to make PDF documents generated by typst compliant with PDF/UA-1 and more accessible in general. Tagging of PDF documents will be enabled by default to ensure a baseline of accessibility and
the--pdf-standard=ua-1 flag can be used in the CLI to enable further checks demanded by the PDF/UA-1 specification. Note however that this cannot guarantee full compliance, since some parts can only reasonably be validated by a human.

Here is a check list currently used for development. Some of the elements are only here for completeness sake and may not require any implementation:

• investigate introspector performance due to increased number of tags
• elements
  • headings
    • initial support
    • generateLbl for numbering
  • outlines
    • generate nestedTOCs for different levels of headings
    • mark fill as artifact
    • generateLbl for numbering
  • tables
    • mark table gutters as artifacts
    • generateTH tags for table header rows
    • groupTHead,TFoot, andTBody row groups.
    • some temporary hacky way to set the table-header-scope on cells
    • ignore repeated headers/footers
    • generateHeaders attribute
    • fix table.header with multiple rows
    • use last repeated footers
  • figure
    • captions
    • BBox and width/height
  • image
    • wrap in figure tag if not already inside one
  • terms
  • lists
  • enumerations
  • bibliography
  • quote
  • ref
  • cite
  • footnotes
  • grid
  • stack
  • columns
  • block
  • box
  • inline
  • shapes, line, path, and curve
  • text layout elements
    • strong, underline, overline, strike, sub, sup, emph, highlight, smallcaps
  • layout elements
    • skew, scale, rotate, move
  • symbol
  • repeat: mark as artifact
  • raw
  • math
    • generate a formula tag
    • allow specifying an alternative description
  • place: possibly interfering with reading order
  • hide: don't generate tags
  • frame
  • links
    • only generate quadpoints for links in PDF/UA mode
      • most pdf readers (even acrobat) don't handle them properly
    • link annotation alt description (Contents attribute) is required
      • generate alt text where applicable
• fix grid layouting issues due toTableCell andGridCell tags
  • somewhat relatedAvoid breaking after an empty frame #6335
• fix overlapping tags generated byPAR grouping rule
• Bbox for bg artifacts
• text handling
  • soft hyphens
  • ensure there is whitespace between words and at the end of lines
• check the reading order of elements
• some form of snapshot testing (maybe a human readable tag tree)