Google Gears is a fantastic browser plugin; it allows a developer to create applications that run while offline, syncing with a server when online. Two great examples of the power of that mechanism are Gmail (both the “desktop browser” and the mobile Android-version) and Mindmeister (only while in trial, for paying Mindmeister-accounts after that period). The problem with Gears however is that it’s a plugin and not a lot of people have it installed: only Chrome-users have it by default. And that’s where HTML5 comes in; one of the areas where the new spec offers vast improvements over html4/xhtml is the ability to take webapps offline by allowing a developer to store files for offline usage and to write data to a local, browser-embedded database. Both Safari 4 and Firefox 3.5 support these features, so maybe HTML5 makes Gears already redundant in those browsers with more to come?
I haven’t gotten around to experimenting with offline databases yet, but I did already look into offline files. At first sight, Gears Localserver and HTML5 Offline Webapps indeed seem very similar; your html-page points to a manifest-file which contains a list of assets (pages, images, css, js, …) that the browser has to store for offline usage. Easy enough, no?
So HTML5 Offline Webapps is no drop-in solution to speed up delivery of dynamic pages, you’ll still need Gears to take care of that (or rely on old-fashioned carefully configured expiry- and cache-headers). But, as Google proves with the iPhone-version of Gmail, Offline Webapps combined with a HTML5 offline database can work miracles if you use it the correct way.