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Hyper Text Language Markup.

Intended misspelling of HTML, a front-end programming language used to define elements in a website. As with many intended misspellings, HTLM was first coined by someone who accidentally misspelled it and was subsequently used to ridicule their mistake.

This acronym is also used as an 'inside joke' amongst designers and coders alike, preferably used when around people that are less knowledgeable in the field of front-end programming.
Matt: You bloody better have that HTLM done by tomorrow!
Dick: Yeh yeh, as long as you do the SSC.
by Kiorrik December 16, 2009