☔ Update website for accessibility (European Accessibility Act)

[atsansone]

This project covers updating the docs.flutter.dev site to meet Google's accessibility standards.

This includes the following tasks:

Assess a11y needs

• Run multipage a11y assessment tool in HTML #9535
• Run multipage a11y assessment tool in JSON #9534
• Aggregate changes from JSON results #9533

Fix site wide common issues

• Request new color set from Design #9532
• Correct CSS to match brand color palette and meet accessibility requirements #9427
• Fix form controls in header file #9523
• Fix duplication of in-page ToC #9707
• Fix duplication of side navigation #9708

Fix heading issues

• Fix empty links #9524
• Fix heading order #9525
• Fix ARIA IDs and attributes #9526
• Add alt text #9527

Fix ARIA IDs and attributes

• Add titles to embedded iframes (YouTube) #9528
• Fix duplicate ID attributes #9529
• Check DOM / tab order #9530
• Sidenav dropdowns aren't updating their expanded state #11811

Fix lexer contrast

• Fix rouge highlighting colors for contrast #9531

Core a11y Testing

• Add title to interactive iframe #10530
• Add Skip to Main Content link #10531
• Fix clipped text in footer in responsive design #10532
• Fix descriptions for Visit <page> links #10533
• Clarify use of certain buttons and links #10534
• Fix list structures #10535
• Silence arrow icons in dropdown menus #10536
• Fix colors in search results page footer #10537
• Fix color contrast in search results #10538
• Verbalize current page of search results pagination #10539
• Fix verbalization of Flutter logos in search results page #10540
• Fix focus in search results sort by dropdown menu #10541
• Add role to search results sort by button #10542
• Contain all page content in landmarks #10543
• Make DartPad example button accessible using keyboard #10544
• Enable verbalization of each line in embedded DartPad #10545