The act of using more advanced HTML tricks simply because it's possible. Some might argue that Dynamic HTML's sole purpose is to provide HTML masturbators new tricks with which to increase the loading times of pages for an effect 95% of said pages' vistors will tire of after a single viewing, if not prior to. Notoriously masturbatory HTML tags include <blink>, <marquee>, and <embed>.
The act of HTML Masturbation actually extends beyond mere HTML now, rendering the term a misnomer. Many webmasters now utilise Shockwave's Flash plugin to create even greater loading times for their visitors, all for a brief, low quality sample of their favorite song and a bloated, spinning logo they created, which will provide a new urge to all those who view it: A want to smash open the monitor, reach through the phone/cable/T1 line and beat the webmaster senseless with his own animated symbols.
As with sexual masturbation, HTML Masturbation isn't something that should necessarily be shared with the world (wide web) and it would likely be for the best if such acts remained within the confines of one's own home.