This session explains the basic concepts in browser implementations of core Web document technologies (HTML, XHTML, CSS, and SVG), explains their general performance characteristics and how they interact with Javascript, and explains common optimizations (such as coalescing changes) that browsers make. Rather than presenting specific techniques authors can use, the goal is to build understanding of the performance characteristics of the Web platform so authors can have ideas of what things are likely to be fast or slow, and what types of changes they can experiment with to improve the performance of their Web pages.