[webcomponents] accessibility enhancements (#340)

Can be explored in good browsers by keyboard-on users, and gives
something useful for more or less all modern screenreader setups
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/340

-- Commit Summary --

  * Merge remote-tracking branch 'w3c/gh-pages' into gh-pages
  * Longdesc content
  * More explorable

-- File Changes --

    A assets/images/composed-tree-desc.html (56)
    M assets/images/composed-tree.svg (275)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/340.patch
https://github.com/w3c/webcomponents/pull/340.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/340