[webcomponents] fixed broken links (#395)

fixed broken links reported by [Link Checker](https://validator.w3.org/checklink)
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/395

-- Commit Summary --

  * fixed broken links

-- File Changes --

    M assets/styles/respec-complement.css (6)
    M spec/imports/index.html (46)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/395.patch
https://github.com/w3c/webcomponents/pull/395.diff

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/395