Definition
The accessibility tree is the structured representation of a page that browsers expose to assistive tech (screen readers). It's derived from semantic HTML + ARIA attributes. Modern AI crawlers increasingly read the accessibility tree instead of (or in addition to) raw HTML, because the tree is already de-noised and structured. A page that's accessibility-broken often has degraded AEO performance too.
An <img> without alt-text creates a gap in the accessibility tree. A <div onClick> instead of <button> creates an unclickable node in the tree.
How to optimize
Pass automated a11y audits (Lighthouse a11y, axe-core). Use semantic HTML by default. Add ARIA only when semantic HTML can't express the role.