CSSStyleDeclaration was backed by an in-memory _props map that was never
bridged to the element's style content attribute. Parsed inline styles were
invisible to el.style.*, setAttribute('style', …) did not update el.style, and
el.style.x = … never reached the attribute or serialization; getComputedStyle
read the same disconnected declaration.
Give the declaration its owner element: lazily pull the style attribute into
_props on read (skipping the reparse when unchanged) and push _props back to the
attribute on every mutation. Owner-less declarations (getComputedStyle fallback,
stylesheet rules) stay purely in-memory, so their behavior is unchanged.
Fixes#488
Andrey Mnatsakanov
80803cbec8