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HDML stands for Herp-Derp Markup Language. Broken or incorrectly implemented HTML that ends up being displayed as garbled text on the page, such as the following:

Show the one you love how you <i>really</i> feel this Christmas!
The frequent occurrence of HDML in the stranger's email body led Randolph Carter to suspect a phishing scheme.
by hypomnesis March 09, 2011
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