Probably does not insert space, but causes surrounding spaces not to collapse:
Usually, two spaces should display as a single one:
but if there is anything between those two spaces, even if it's something with "display: none", then they are not two consecutive spaces anymore, and they are displayed as a wider space:
Code:
two <span class="pagenum">123</span> spaces
(where "span.pagenum" is set to "display: none").
In this case, it would be better to include one of the spaces inside the span:
Code:
two <span class="pagenum">123 </span>spaces
But it may still affect searching. If you search "two words" you may find no match. I'm not sure what the specifications say, if they say anything, about these things.