Identifies HTML script tags that don't have defer or async attributes.

When neither of those attributes are used, a script tag blocks the construction and rendering of the DOM until the script is loaded, parsed and executed. It also creates congestion on the network, messes with the resource priorities and significantly delays the rendering of the page.

Considering that JavaScript on Shopify themes should always be used to progressively enhance the experience of the site, themes should never make use of parser-blocking script tags.

As a general rule, use defer if the order of execution matters, and use async if it doesn't.

Learn more about improving your theme's performance.