One of the main features of the Jetpack Boost plugin is called “Optimize CSS Loading”. When you enable this feature, the plugin figures out the most important CSS rules to display first (called Critical CSS) for different sections of your website. This Critical CSS helps pages display your content sooner, so your users don’t have to wait for the entire page to load.
When you enable “Optimize CSS Loading”, the Critical CSS generation process starts, and a green progress bar is shown. If you open your browser’s developer tools, you may notice a few errors depending on the browser you are using.
Here are some examples:
Behind the scenes, the plugin opens sample pages from your site in a hidden iframe to assist with generating the required Critical CSS. The iframe is configured to use tight security settings, which block any script execution from inside the iframe. As a result, many browsers report blocked content in their console.
Since these are safety measures to protect your site; you can safely ignore them!