html-tidy takes care of some post-processing, rendering other
substitutions obsolete. Remove the obsolete regex substitutions.
Now that we did that, the remaining substitutions can be done with
vanilla POSIX or POSIX-Extended regular expressions. Replace sd with
sed, and group the substitutions together into one invocation instead of
multiple invocations piped together. This change speeds up
post-processing to be almost as fast as the initial build step.