Uses document.write()

Using document.write() can delay the display of page content by tens of seconds and is particularly problematic for users on slow connections. Chrome therefore blocks the execution of document.write() in many cases, meaning you can't rely on it.

In the Chrome DevTools Console you'll see the following message when you use document.write():

[Violation] Avoid using document.write().

In the Firefox DevTools Console you'll see this message:

An unbalanced tree was written using document.write() causing
data from the network to be reparsed.

How the Lighthouse document.write() audit fails #

Lighthouse flags calls to document.write() that weren't blocked by Chrome:

For the most problematic uses, Chrome will either block calls to document.write() or emit a console warning about them, depending on the user's connection speed. Either way, the affected calls appear in the DevTools Console. See Google's Intervening against document.write() article for more information.

Lighthouse reports any remaining calls to document.write() because it adversely affects performance no matter how it's used, and there are better alternatives.

Avoid document.write() #

Remove all uses of document.write() in your code. If it's being used to inject third-party scripts, try using asynchronous loading instead.

If third-party code is using document.write(), ask the provider to support asynchronous loading.

Resources #

• Source code for Uses document.write() audit
• Intervening against document.write()
• Parser blocking versus asynchronous JavaScript
• Speculative parsing