Bug 2008041 made disabled, checked, hidden, collapsed, and selected
boolean attributes, so their value is empty and [disabled="true"] no
longer matches. Nothing sets any of them to "false", so matching on
presence alone is equivalent.
- right/left arrows on section header expand/collapse the sections
- space/enter on section header toggle section collapsed/expanded
- tab from the header goes through the header's buttons and then
tabs into the section if that's opened
- arrow up/down on the header jumps to the previous/next header
- space/enter on clickable elements simulate a click
- if there is a pinned section, tab from the title will focus its header
