If you want this PR to be considered, please explain in words what it is doing and why.
A link to a huge discussion isn't enough. The explanation should also go in the commit message.

If you want this PR to be considered, please explain in words what it is doing and why. A link to a huge discussion isn't enough. The explanation should also go in the commit message.

I find the following html

<table>
<caption>...<span id="test">test</span>...</caption>
...

would result in latex a \phantomsection in caption, which causes an error.
And the link above gives a solution.

Complete example:

<table>
<caption>...<span id="test">test</span>...</caption>
<tr><td>A</td><td>b</td></tr>
</table>

gets converted to

\begin{longtable}[]{@{}ll@{}}
\caption{...\phantomsection\label{test}{test}...}\tabularnewline
\toprule\noalign{}
\endfirsthead
\endhead
\bottomrule\noalign{}
\endlastfoot
A & b \\
\end{longtable}

and compiling this produces the LaTeX error

! Undefined control sequence.
\Hy@MakeCurrentHref ...def \HyperLocalCurrentHref 
                                                  {#1}\@onelevel@sanitize \H...
l.65 ...n{...\phantomsection\label{test}{test}...}

Adding \protect before \phantomsection makes this error go away.

What isn't clear to me is whether there would be any drawbacks if we add \protect before \phantomsection in every context (not just inside a caption).

It seems to work in my tests with \protect in every context, so I think we can do this.