HTML: Custom Tag (Custom Element)

The HTML standard has a concept of custom element. That is, any arbitrary tag (with some naming requirement), like this

<abc-xyz>custom element</abc-xyz>

you can use CSS to define its appearance and use JavaScript to define its behavior.

Custom element name requirement

All characters must be lowercase.

1. Start with lowercase a to z
2. Followed by zero or more simple Unicode character
3. Followed by a hyphen
4. Followed by zero or more simple Unicode character

the “simple Unicode character” includes Emoji 😄

Reference

https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements