Optional line-breaking HTML entity that is always invisible

Solution 1:

​ is the HTML entity for a unicode character called the zero-width space (ZWSP).

"In HTML pages, this space can be used as a potential line-break in long words as an alternative to the <wbr> tag."- Zero-width space - Wikipedia

The <wbr> tag also works, as mentioned by Aaron's answer. I think I prefer the HTML entity over the tag because the entity seems simpler: unicode handles it, not the web browser.

Solution 2:

<wbr> looks like it does what you want, but it looks like the support for it, and ­ for that matter, is very inconsistent. So unfortunately, there may not be a particularly good way to do what you want.