The summary.
BUILDING A DIALOG OR MENU CORRECTLY IS HARDER THAN IT APPEARS
Focus trapped and restored, escape closing it, announced to assistive technology, keyboard operable throughout.
Most hand-built versions get all of that wrong. Adopt a library for complex interactive components.
EVALUATE ACCESSIBILITY AS TESTED, NOT CLAIMED
Along with customisability, bundle size, maintenance activity and documentation.
THEME A LIBRARY THROUGH ITS SYSTEM, NEVER BY OVERRIDING
Overrides fight the library and break on upgrades. And check that unused components are excluded from the build.
WRAP LIBRARY COMPONENTS RATHER THAN USING THEM DIRECTLY EVERYWHERE
It gives one place to apply conventions, and makes replacement possible later.
UNSTYLED ACCESSIBLE PRIMITIVES GIVE YOU THE HARD PART WITHOUT A VISUAL OPINION
WHAT ORGANISATIONS UNDERESTIMATE ABOUT AN INTERNAL LIBRARY
The ongoing cost: ownership, documentation, versioning, releases and accessibility expertise.
DOCUMENT WHEN NOT TO USE A COMPONENT
Without that, it gets used everywhere. And a library nobody can browse is not used.