As you already read from the comments, there is no HTML version for the manual (not for TikZ and not for pgfplots).
But since many browsers use pdf.js as default viewer for PDFs, you may be able to link to internal links.
http://pgfplots.sourceforge.net/pgfplots.pdf#pgfp.axis (links to the axis environment) or
http://pgfplots.sourceforge.net/pgfplots.pdf#pgfp./pgfplots/surf (links to the definition of the
The same is also possible for TikZ. These anchors are stable, also between versions. They belong the internal cross-referencing system.
I believe that this would be address your use-case up to the fact that
one may need to ensure that pdf.js is used, not some other pdf viewer
the loading times of pdf.js are too long for these manuals. A real HTML version could make use of caching.
As users stated in comments, generating a "real" HTML version is a quite involved task. I have also spent considerable effort in an HTML version of the pgfplots manual, with limited success.