What are "data-require" "data-semver" in HTML files?
I was curious about this as well. These appear to be data- attributes that are created by plunker when including external libraries in its editor. Since plunker is used heavily throughout the development community that is probably why you see it appearing a lot.
By themselves they don't do anything special, unless a library or service decides to use them.
The 'data-semver' attribute appears to be an implementation of Semantic Versioning
To the best of my knowledge no libraries actually use them for any sort of dependency checking/loading.