A Document document allows adding render-blocking elements if document's content type is "text/html" and the body element of document is null.

A Document document is render-blocked if both of the following are true:

• document's render-blocking element set is non-empty, or document allows adding render-blocking elements.

• The current high resolution time given document's relevant global object has not exceeded an implementation-defined timeout value.

To block rendering on an element el:

1. Let document be el's node document.

2. If document allows adding render-blocking elements, then append el to document's render-blocking element set.