Epic: accessibility

[cscheid]

• Keyboard navigation order for website navigation area #7058
• Insert SVG file as <object> instead of <img> in HTML (better for accessibility) #6581
• a11y: tooltips interact badly with voiceover #6500
• Left-nav menu expand/collapse should be buttons/use disclosure pattern (a11y issue) #4935
• Collapsed callouts using invalid ARIA attribute (a11y issue) #4934
• Support PDF/A-compliant output using pdfx package #4426
• [Accessibility] fig-alt does not work for mermaid #3873
• Add follow hint to collapsed section #2023
• Accessibility suggestions: Footers, table of contents, and headers #1032
• Unused links in HTML output of code chunks #1030
• Improve table markup for accessibility (a11y issue) #1025
• Implement some sort of accessible/text-alternative for callout icons or blocks (a11y issue) #726
• Add valid labels for search button and input fields (a11y issue) #678