There's no custom syntax, they only use existing HTML language features, namely element attributes. Custom syntax would be something that is not already a synta

There's no custom syntax, they only use existing HTML language features, namely element attributes. Custom syntax would be something that is not already a syntax, i.e., language feature, in HTML. And why exactly should this not be checkable at build time? The attributes are defined, their use too, it's as checkable as any JavaScript framework. If you're using TypeScript, or the like, then you'll naturally get much more things that can be checked, but those are type checks and not syntax checks, i.e., something completely different. Don't get me wrong, personally I'm all for static typed, and schematic frameworks but that's completely orthogonal to using (custom) HTML attributes, and has nothing to do with syntax checks. I mean, if syntax could not be checked it couldn't be parsed at all...