In certain cases an aggregate of HTML documents makes sense.
This is especially true if those files are supposed to be sent delivered via other means than http[s], e.g. for offline manuals or simple forms.

For this case there's the MHTML mime type which - AFAIK - isn't supported by all browsers yet.
https://en.wikipedia.org/wiki/MHTML

Of course there are workarounds, that are supported by almost all browsers (e.g. using base64 encoding, embedded CSS, etc.), but MHTML makes things way simpler and modular.