RDBMSers love their ACID and SQL, but the rapidly-growing world of in-browser databases can't afford, and usually doesn't need, that kind of pristine simplicity.

For web applications, even database-centered applications, performance and ease of use -- and, most of all, native comfort with JavaScript -- are far more important. Browsers are platforms, but still not whole machines. Scaling down a SQL-queryable database, as the old WebSQL standard did, is not the optimal solution.

IndexedDB is, says W3C; but because its intended audience is composed of web developers, not (chiefly) database developers, IndexedDB coding feels a lot more like DOM work than hard-core database development.

This tutorial from html5rocks.com is written to ease the database-to-webapp transition.

• You'll see the WebSQL process first, for most basic CRUD-type processes -- most of the meat here is in the SQL itself.
• Then you'll be see the IndexedDB equivalent, right below -- which will immediately look more familiar to the JavaScript (or other OO) developer, and incidentally perhaps a bit friendlier to the NoSQLer as well.

The top of the guide also provides a helpful side-by-side (columnar) comparison of WebSQL and IndexedDB, offering a more bird's-eye and standards-centric view than the code samples that make up the bulk of the article.

Definitely worth reading/bookmarking, if your web app needs to run a database in browser.