As far as I can tell, only two browser engines ever implemented WebSQL: Safari and Chrome's WebKit (which hadn't yet been forked into Blink), and Opera's Presto

As far as I can tell, only two browser engines ever implemented WebSQL: Safari and Chrome's WebKit (which hadn't yet been forked into Blink), and Opera's Presto (which hadn't yet been abandoned in favor of Chromium). Firefox never implemented it, and neither did IE (Edge hadn't been announced yet), so it certainly wasn't implemented by "all vendors". And both implementations were just thin wrappers around the same library, SQLite, so the W3C was correct in saying there were no actual independent implementations.