[w3c/manifest] Fix validator errors and warnings (#625)

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/625

-- Commit Summary --

  * Fix validator error
  * Fix validator warning

-- File Changes --

    M index.html (19)

-- Patch Links --

https://github.com/w3c/manifest/pull/625.patch
https://github.com/w3c/manifest/pull/625.diff

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