Implementing WebSQL was so much work that all the browser vendors just basically bundled SQLite and called it a day. The problem was that the working group required at least one other implementation, before it became a standard. But that was such a monstrous project that none of the browser vendors wanted to take it on.
What the browser vendors agreed on is that they would rather have IndexedDB. I've never used it, but they say it is a lower-level API than SQL, and using it you could build your own SQL abstraction layer above it.
So we have as standards the key-value-based localStorage and then, halfway between that and SQL, is IndexedDB.
I participated in HTML5 specification work group at W3C as Invited Expert. We had such a proposal to add SQL storage to HTML5. But it was pretty much unanimous opinion that adding specification of any SQL flavor to the HTML spec would be too much.
Yeah, basically it would've had to specify all of SQL, or to say "do what SQLite does", and neither option was very palatable to all the parties. Some browsers [1] did implement WebSQL before it was formally abandoned, though.
Some browsers can use Open Source libraries, some simply cannot , for whatever reasons. Yet formally we need at least two independent implementations of the same thing in order the spec to get Recommendation status.
So each UA (a.k.a. browser) has to implement any SQL spec from scratch adding its own SQL flavor.
In fact I am pretty sure that it is possible to add some basic SQL features by JS on top of IndexedDB. It should be such things already I think.
What the browser vendors agreed on is that they would rather have IndexedDB. I've never used it, but they say it is a lower-level API than SQL, and using it you could build your own SQL abstraction layer above it.
So we have as standards the key-value-based localStorage and then, halfway between that and SQL, is IndexedDB.
More: https://hacks.mozilla.org/2010/06/beyond-html5-database-apis...