Revert a minor grammar mistake just introduced

This was actually correct before commit https://github.com/playframework/playframework/commit/57046be7a1078ba9108fb5cf06e61d36f9f540fa. You use "an" before a vowel sound not just before an actual vowel. When you pronounce HTML, the first sound is 'a' or 'eh'. You use "an" so as not to have two vowel sounds in a row.

The APA is one of the most authoritative source on grammar rules. School children and professionals in the social sciences in the US are often mandated to follow the APA style guide.
http://blog.apastyle.org/apastyle/2012/04/using-a-or-an-with-acronyms-and-abbreviations.html

Google also suggests that more informal language usage also prefers "an" over "a":
"an html document" -> 10,100,000 results on Google
"a html document" -> 3,510,000 results on Google.