Apparently HTML5 isn't a tag. Should it be created?
Your original question isn't actually about HTML5 (or Drupal actually, it's about a relative path issue), you just happen to be using an element that wasn't available before that version.
I don't think adding a tag is a good idea since it doesn't apply to the question. If there are (let's say) half-a-dozen or more other questions that are clearly about HTML5 in conjunction with Drupal, then yes, it would make sense to add it. I haven't come across any such questions, if you have some examples please edit them into the question.
I don't think we should create the tag. Essentially, because we actually have a specific close reason for that type of question
Questions on programming, PHP, SQL, etc. that do not relate directly to Drupal are off-topic here, but can be asked on Stack Overflow.
HTML would fit nicely into that list, as would HTML5. If a question really is about html 5, it's actually off-topic on the site! So no, I don't think the tag should be created.
In addition, had the tag truly been needed, it would have organically been created anyway. There's almost 1000 users with high enough reputation to create the tag, and those users, (since they have higher rep) are the most active on the site. Despite that, not one of them appears to have had need for the tag.