This is not a parser issue.

Better solution:

.tocnumber {
	display: table-cell;
	padding-right: 0.5em;
}
.toctext {
	display: table-cell;
}

Change 196541 had a related patch set uploaded (by Gerrit Patch Uploader):
TOC: Separate columns for tocnumber and toctext

https://gerrit.wikimedia.org/r/196541

Screenshots with Firefox

Before:

After:

Change 196541 merged by jenkins-bot:
TOC: Separate columns for tocnumber and toctext

https://gerrit.wikimedia.org/r/196541

If users have underlining for links enabled in their preferences, and are using Firefox (maybe other browsers too), the table-cell styling of the inner span's is blocking inheritance of the underline styling from the links apparently.

I see some info on MDN, indicating that this is quirks mode behavior (or at least was at some point). Adding text-decoration: inherit to the same lines would restore normal behavior.

Change 251565 had a related patch set uploaded (by Gerrit Patch Uploader):
TOC: Workaround for missing underlines in Firefox

https://gerrit.wikimedia.org/r/251565

http://www.sitepoint.com/web-foundations/text-decoration-css-property/

Any text decoration setting on a descendant box can never “undo” the text decorations of an ancestor box.

Paraphrasing from http://stackoverflow.com/posts/4377895/revisions

For any browser except IE: just add display:table; to the child's style to cancel out an ancestor's text-decoration. For some reason in FF you can use any of table, block, list-item or table-caption but these don't work in Safari/Chrome.

But then it's either a bug, or no longer a quirks mode thing as far as I can tell. Been trying to find more clues in the specs, but haven't been able to find any confirmation one way or the other.

Filed https://bugzilla.mozilla.org/show_bug.cgi?id=1222703 :)

Setting table-caption also has significant side effects if someone wrapped the TOC inside a table...
Perhaps we should just revert...

@TheDJ Can you give an example?

@Fomafix https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&oldid=689650872#TOC

The problem of increased spacing when TOC is in a table can mitigated by D135.

More downstream problems due to changed TOC: https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Problem_with_Horizontal_TOC_on_Chrome

In T92481#1792001, @TheDJ wrote:

Setting table-caption also has significant side effects if someone wrapped the TOC inside a table...
Perhaps we should just revert...

I'm all for that. This "solution" was poorly conceived first because span.tocnumber and span.toctext are both wrapped by a single <a href=#...> anchor tag (all inline level elements) to begin with plus the overlooked additional pseudo ::after entry inserting a "space" between those two spans set to display:inline-block. Please refer to the following to see this illustrated in html.

<div class="toc" id="toc">
<div id="toctitle">
  <h2>Contents</h2>
  <span class="toctoggle">&nbsp;[<a id="togglelink" href="#">hide</a>]&nbsp;</span>
</div>
<ul>
<li class="toclevel-1 tocsection-1">
  <a href="#Copy_and_paste_table_using_VE">
    <span class="tocnumber">1</span> <span class="toctext">Copy and paste table using VE</span>
  </a>
</li>
<li class="toclevel-1 tocsection-2">
  <a href="#Help_in_fixing_a_cite_error">
    <span class="tocnumber">2</span> <span class="toctext">Help in fixing a cite error</span>
  </a>
</li>
. . .
</ul>
</div>

Changing the display behavior of the spans to table cells "splits" the anchor and violates the spec. -- if anything the anchor should be re-defined as either display: table-row or maybe even display: inline-table.

The bigger problem with the change to display:table-cell is the space between the spans being generated by...

.tocnumber::after {
    content: " ";
    display: inline-block;
    width: 0.5em;
}

That - in effect - makes the composition of the "faux table row" (the <a> anchor) a table-cell (span.tocumber), an inline block (the space inserted ::after span.tocnumber) and another table-cell (span.toctext). All three should be inline and/or inline-blocks OR all table-cells using 3 spans instead of two (dropping the ::after insertion for parser generation of a middle [3rd] span).

I have other points with the css for .toc/#toc but they aren't directly related to the problems introduced with the change to table-cell 'display'

Change 251565 merged by jenkins-bot:
TOC: Restore missing underlines in Firefox

https://gerrit.wikimedia.org/r/251565