Mozilla Notes on HTML Reflow

Reflow is the process by which the geometry of the layout engine's formatting objects are computed. The HTML formatting objects are called frames: a frame corresponds to the geometric information for (roughly) a single element in the content model; the frames are arranged into a hierarchy that parallels the containment hierarchy in the content model. A frame is rectangular, with width, height, and an offset from the parent frame that contains it.

Stubbornella » Blog Archive » Reflows & Repaints: CSS Performance making your JavaScript slow?

Going forward the performance community needs to partner more with browser vendors in addition to our more typical black box experiments. Browser makers know what is costly or irrelevant in terms of performance. Opera lists repaint and reflow as one of the three main contributors to sluggish JavaScript, so it definitely seems worth a look.