HTML5, or perhaps better termed "emerging web technologies" are an incredibly heavy stack. You have 3 different means of markup being parsed from the get-go: HTML/SGML, XML and JSON. HTML is a quite lightweight component, OK. But then you get CSS. CSS used to be simple, but it's getting incredibly complex with tons of new standardised features and vendor prefixes. Initially it just drew lines and boxes in colour with text, positioned correctly on the page according to a few simple positioning systems. Now it draws coloured boxes and lines with 3d transformations, animated, with alpha transparency, a drop shadow and rounded corners, at the same time as rendering text, and positioning them correctly on the page using several different layout systems. Then, there's the image formats commonly supported. PNG, JPEG, GIF, and ICO (Windows, favicons). SVG, a fully-featured vector graphics format which also uses CSS, as well as having its own, different animation system, and its own scripting system! Then we have common audio and video formats. MP3, WAV, OGG, MP4/H264 and WEBM. As well as the subtitle formats. Oh yeah, MathML parsing is in there somewhere too.

And then we have Javascript, a simple, not terribly complex language. OK. But we also have tons of APIs for it. The Document Object Model API. The XMLHttpRequest API. The Canvas API - a complete 2d rendering context AND full OpenGL ES 2.0. WebSockets. Local storage and session storage. IndexedDB. Game controller API. Page visibility API. Audio composition API (forgot proper name). Touch events. BlobBuilder. Geolocation. History API.

Just to name a bit of it.