What is HTML “is” attribute?

In 2020:

The is attribute is now part of HTML spec in the Custom Elements specification.

It follows the polymer spec and is documented for developers at mdn.

Only Edge still hasn’t updated to include this spec but it its new chromium-based implementation, in 2020, its availability may become widespread.

In 2017:

There is no is attribute in HTML.

It is a proposed extension that appears in the Custom Elements specification (which evolved from the Polymer spec mentioned below).

It allows you to say that an existing, standard element is really a custom element.

<button is="fancy-button" disabled>Fancy button!</button>

… which allows for backwards compatibility. If custom elements are not supported by the browser (the spec is still a draft and has very limited browser support) then it will fall back to the default behaviour.

In 2014:

It is not HTML. It is an expando-attribute for Polymer custom elements.

If you used extends to create a Polymer element that derives from an existing DOM element (something other than HTMLElement), use the is syntax