I've noticed that the more experienced a front end developer is, the less HTML they write to achieve their desired result.
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Today I learnt that you can mix HTML inline in markdown! For example, the following is valid.
Foo <hr/>
I'd assumed that you needed HTML separately, like ``` blocks, but no: https://spec.commonmark.org/0.31.2/#raw-html
Admittedly HTML is very restricted on most sites, but it's helpful for SSGs.
I've heard of 'blub languages', where you don't realise that other languages have better abstractions until you've experienced them.
I think the same thing happens with individual features. I've seen several C++ folks miss variadic generics in Rust, but I've not written enough C++ to feel it.
One cute feature of markdown I'd not noticed before: there's no syntax for images *without* a description (i.e. alt text).

https://spec.commonmark.org/0.31.2/#images