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

The IDL (Interface Definition Language) 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, the following JavaScript:

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

Is equivalent to setting the class content attribute.

In JavaScript texts, the IDL attributes are often referred to as properties because they are exposed as properties of DOM objects to JavaScript.

While there's usually a corresponding pair of a content attribute and an IDL attribute/property, they are not necessarily interchangeable. For example, for an <option> element:

• the content attribute selected indicates the initial state of the option (and does not change when the user changes the option),
• the property selected reflects the current state of the control