Validity and compatibility fixes for themes/notmyidea (including testsuite adjustment)

This is a rework of my other pull-request, considering your suggestions.
I am sorry that I neglected to read the contribution guidelines.
This pull-request should meet your needs.

* Missing ol tags around li in authors.html and tags.html.
* Stray ol and section end tags if only one article is displayed (index.html).
* Add an explicit space after a link to a tag such that adjacent
  tags do not get concatenated by Lynx.
* Simplify an unnecessarily complicated conditional.