Why is the rendering of HTML a cpu intensive process?

It's been often stat开发者_JAVA百科ed that the gating factor in html rendering performance is cpu. I've also seen this by individually increasing cpu speed, amount of memory and disk speed and the only change that impacts html rending is cpu,

Why is the rendering of HTML a cpu intensive process (or said a different way, why is cpu speed the gating factor in html rendering)?

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Calculation of dimentions, spaces between elements, drawing small images, all that occurs while the browser is still loading the HTML page and maybe other resources, which also means several repositioning of those loaded elements. In very few situations you may get high memory page faults if large elements are found such as large SWF files or giant uncompressed images. But overall it's a process of calculation and recalculation, which CPU is the unit for the task.