Should CSS always precede JavaScript?

In the realm of web development, the placement of CSS and JavaScript resources within the HTML document has been subject to ongoing debate. Traditionally, it has been recommended to include CSS before JavaScript due to the belief that the rendering thread requires all necessary styling information before proceeding. However, recent advancements in browser technology have challenged this notion.

Testing the Assumptions

A detailed test was conducted to validate the assumption that CSS must precede JavaScript for optimal performance. The test involved:

Page with CSS included before JavaScript (CSS First)
Page with JavaScript included before CSS (JavaScript First)

Both pages included delayed loading of JavaScript and CSS resources to simulate realistic conditions. The test was repeated multiple times in different browsers.

Results on Desktop Browsers

CSS First: Delays in CSS had a negligible impact on page load time.
JavaScript First: When CSS was delayed, the page loaded significantly faster, indicating improved performance.

Conclusions for Desktop Browsers

Contrary to the traditional recommendation, placing external style sheets after external scripts may provide better performance. This is because modern browsers implement speculative parsing, where resources are downloaded in parallel regardless of the order in which they appear in the HTML.

Mobile Browsers

The test was also conducted on an Android device:

JavaScript First: Loading CSS was observed to be delayed until JavaScript execution was complete, suggesting that support for speculative parsing may be limited on some mobile browsers.

Browser Support

Speculative parsing is supported by the majority of modern browsers, including Chrome, Internet Explorer 8 , Firefox 3.5 , and Safari 4 . This means that for most desktop users, putting JavaScript before CSS provides a performance benefit.

Consider Mobile Constraints

On mobile devices, the performance impact of placing JavaScript before CSS may vary depending on the browser and device capabilities. Developers should consider optimizing for mobile users by loading CSS first, as speculative parsing may not be fully supported.

Additional Notes

The placement of scripts within the is assumed, as opposed to the end of the .
The results apply to external style sheets and scripts. Inline CSS and JavaScript may follow different performance considerations.
Other factors, such as asynchronous loading, external service calls, and browser caching, can impact overall page performance.

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