Improve HTML style guide

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Follow up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17616#note_63407279

Two classics that might make sense in a future iteration:

• Put paragraphs in p and other rules about headlines (especially in conjunction with our new typography)

• no multiple br or br for alignment

Also try to reduce Dom elements + nesting especially in repeated blocks to improve scrolling performance.