Focus management and tab navigation #150

Summary#

Implement focus tracking, tab-order navigation, and focus-related events and pseudo-classes.

Details#

Focus tracking#

• Document.activeElement tracks the currently focused element
• Only focusable elements can receive focus (form controls, elements with tabindex, <a> with href)
• Clicking a focusable element moves focus to it
• element.focus() and element.blur() methods

Tab navigation#

• Tab key advances focus to next focusable element in tab order
• Shift+Tab moves focus backwards
• Tab order: elements with positive tabindex first (ascending), then DOM order for tabindex=0 and naturally focusable elements
• Elements with tabindex=-1 are focusable via script but skipped in tab order

Events#

• `focus` / `blur` events (don't bubble)
• `focusin` / `focusout` events (bubble)

CSS pseudo-classes#

• `:focus` matches the focused element
• `:focus-visible` matches when focus should be visually indicated (keyboard navigation)
• `:focus-within` matches ancestors of the focused element

Acceptance criteria#

• `document.activeElement` correctly tracks focus
• Tab/Shift+Tab cycles through focusable elements in correct order
• `tabindex` attribute is respected for ordering and focusability
• Focus/blur events fire correctly with proper bubbling behavior
• `:focus`, `:focus-visible`, `:focus-within` pseudo-classes match correctly in selector matching
• Focus ring renders around the focused element
• Tests cover tab order, event dispatch, and pseudo-class matching

Dependencies#

• Form element parsing and DOM interfaces