What is the “data-sigil” HTML attribute?

Question

I just came across a website that has \"data-sigil\" as an attribute for HTML elements like a, div, label, abbr.

Answer 1

It's actually a sigil from Javelin (Facebook JavaScript framework). Javelin separates style markers (ids and classes) from functional markers. They use sigils for the latter. See:

http://phabricator.com/docs/javelin/article/Concepts:_Sigils_and_Metadata.html

Answer 2

Any attribute starting with data- is an HTML 5 data- attribute.