How do you add extension to markdown/commonmark? There's no official extension syntax, but custom URIs is elegant:
miniblog.
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One cute feature of markdown I'd not noticed before: there's no syntax for images *without* a description (i.e. alt text).

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 love how the CommonMark Spec has a test suite that's just a JSON array. It's really easy to test a library for compliance, and I've seen developers nerd-sniped into full compliance.
https://spec.commonmark.org/0.31.2/spec.json
