What does idl attribute mean in the W3C html5 standard document?

The 'IDL' comes from the Web IDL spec:

This document defines an interface definition language, Web IDL, that can be used to describe interfaces that are intended to be implemented in web browsers. Web IDL is an IDL variant with a number of features that allow the behavior of common script objects in the web platform to be specified more readily. How interfaces described with Web IDL correspond to constructs within ECMAScript execution environments is also detailed in this document.

Content attributes are the ones that appear in the markup:

<div id="mydiv" class="example"></div>

In the above code id and class are attributes. Usually a content attribute will have a corresponding IDL attribute, for example doing this from JavaScript:

document.getElementById('mydiv').className = 'example'

Is equivalent to setting the class content attribute.