This.

I've long ago abandoned spaghetti mega-scripts and SPAs in favor of targeted JS libraries that extend HTML capabilities. They're not web components, they don't require some framework to manage, and my HTML works fine without them, but if I load the library and add certain attributes or tags to my markup, I get extra capabilities. The libraries don't require crazy tooling to manage, don't have extensive dependencies and so they facilitate steady incremental development.

I'm very happy with this approach and I'm reusing code I've written years ago for completely different projects.