Even though I'm a JS-first dev but I second that. ( HTML & CSS in JS ) is a bad idea and not considered to be progress from the messy yet working templating engines (JS in HTML).

Proponents love to argue that integrating HTML & CSS into JS makes it easier for testing since JS is amenable to thorough testing as opposed to HTML & CSS.

While this is partially true and template files are not straightforward to test esp when you throw app logic into the mix but it still manageable if you kept to a minimum to the view part only and don't go overboard with it.

Also and this is very important, It's a given that HTML & CSS are forgiving compared to JS when it comes to errors but that doesn't mean that it's unlikely for a developer to shoot himself in the foot or to introduce unwanted/unintended bugs in the app through them esp. CSS and these usually are harder to catch and fix.