indefinite articles - A XHTML document or An XHTML document

When writing this sentence:

So it's possible to do an XHTML quine, but not a HTML quine

I noticed that I used an for XHTML but not for HTML. In the discussion on this thread, I've picked up on a few points:

Use an when the following sound is a vowel-sound. The letter X is
pronounced eks ... so an eks-em-el code.

And,

XML code, by the way, is not singular. "An XML code" sounds as odd as
"a C code" or "a Visual Basic code". You could say "an XML tag" but
"XML code" refers to any quantity of code written in XML.

As well as,

1. XML stands for "Extensible Markup Language." (not "extended")

Therefore it seemed more natural to type an for something that's singular (document) and begins with an x. It seems like the "ex" in "extensible" is similar to the "ex" in "extra", since you pronounce the consonants in the XML acronym. Is it incorrect to use an in this case?

Answer

Generally, you use "a" or "an" based on the way you pronounce the acronym (and not its expanded form). The fact that "ex" is the start of either "extensible" or "extended" has nothing to do with the way the letter X is pronounced (which also turns out to be "ex").

So if you pronounce it "ex-aitch-tee-em-ell document", prefix it with "an" (an XHTML document).

Similarly for HTML, "aitch-tee-em-ell" still starts with a vowel, so you'd use "an" for it as well.

So this form:

So it's possible to do an XHTML quine, but not an HTML quine

would be correct.

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Now, an acronym like SQL is more contentious, because "ess-cue-ell" and "sequel" are two ways to pronounce it, one with a vowel and one without, so "a SQL database" and "an SQL database" are both valid depending on how you pronounce it.