Not exactly "hidden", but the two new features in the article are soft hyphens (for automatic mid-word line breaking) and CSS display:inline-block. Good stuff, although it's a shame that it hasn't been supported earlier, the Mozilla team have been slow to catch up on these.
The soft hyphens are going to be tedious in practice if you're going to have to insert them manually. LaTeX does automatic breaking, has anyone used any stand-alone libraries that will insert the soft hyphens into a given chunk of text automatically? The rules for this are different for each (human) language, so it's not a 100% trivial problem; in some cases soft hyphen support alone might not be enough, such as in case of the now obsolete German way of breaking words containing ck into k-k. (the new rule is that the ck isn't broken up and moved onto the new line entirely, so soft hyphens will work now)
The soft hyphens are going to be tedious in practice if you're going to have to insert them manually. LaTeX does automatic breaking, has anyone used any stand-alone libraries that will insert the soft hyphens into a given chunk of text automatically? The rules for this are different for each (human) language, so it's not a 100% trivial problem; in some cases soft hyphen support alone might not be enough, such as in case of the now obsolete German way of breaking words containing ck into k-k. (the new rule is that the ck isn't broken up and moved onto the new line entirely, so soft hyphens will work now)