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.
Examples are:
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 surf macro).
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.
doc/generic/pgf/version-for-tex4ht/en/, but the compilation failed in a lot of points... maybe a need a much more modern system, I don't know. – Rmano Feb 17 '16 at 20:56pdftohtml, no way... – Rmano Feb 18 '16 at 11:37tex4ht, I could offer a site to put it... but till now I failed. – Rmano Feb 19 '16 at 15:33pgfmanual last summer, it is pretty hard thing. Some configurations fortex4htare provided for the manual, but they obviously don't work with the current version. I've fixed some issues, but I still wasn't successful, mainly because of the huge size and large number of errors which always halted the compilation. But I was able to compile some chapters. – michal.h21 Feb 19 '16 at 20:47