render blocking resources

Before the browser can render a page it has to build the DOM tree by parsing the HTML markup. Whenever the parser encounters a script it has to stop and execute it before it can continue parsing the HTML. In the case of an external script the parser is also forced to wait for the resource to download, which may incur one or more network roundtrips and delay the time to first render of the page. By default CSS is treated as a render blocking resource as well. Make sure to keep your CSS clean, deliver it as quickly as possible, and use media types and queries to unblock rendering.

Scripts that are necessary to render page content can be inlined to avoid extra network requests, however the inlined content needs to be small and must execute quickly to deliver good performance. Scripts that are not critical to initial render should be made asynchronous or deferred until after the first render.

External blocking scripts force the browser to wait for the JavaScript to be fetched, which may add one or more network roundtrips before the page can be rendered. If the external scripts are small, you can inline their contents directly into the HTML document and avoid the network request latency.