If you compress the HTML, which you want to do anyway for HTML/CSS if you care about file sizes, then most of the base64 overhead goes away:

    $ head -c 1000000 /dev/urandom | base64 -w0 | gzip | wc -c
    1009042

    $ head -c 1000000 /dev/urandom | base64 -w0 | zstd | wc -c
    1000300

Then the other question is why this proposed packed format is better than the dozen already existing formats like Web Archive, CHM, MAFF, MHTML, etc.