CSS selector support and silent failures

Supported

Type (Button, dashed forms like stack-layout), .class, #id, child >, descendant, :not(), :is(), :where(), :pressed, :hovered, @media (nestable), and attribute selectors with every operator: [a], [a=v], [a~=v], [a|=v], [a^=v], [a$=v], [a*=v]. Attribute selectors match arbitrary properties — including plain expando props you set from code (btn['testAttr'] = 'x').

Not in the framework's own test surface (treat as unverified): !important, z-index interplay, transforms via type selectors, sibling combinators (+, ~).

Specificity notes

• :where() has zero specificity — #myButton { color: green } beats Button:where(#myButton) { color: red }. Use :where() to write overridable defaults.
• Property names are case-insensitive (cOlOr: blue applies); keyword values generally are not — keep values lowercase.

Silent failure modes (the debugging checklist)