Does $(document).ready() Ensure CSS is Fully Loaded?

Is $(document).ready() Also CSS Ready?

As a programmer, you undoubtedly utilize the $(document).ready() event to initiate scripts that manipulate the DOM upon page load. However, in certain instances, you may encounter unexpected behaviors such as lag in element alignment or incorrect calculations. This arises from the fact that $(document).ready() does not guarantee synchronization with the complete loading of external CSS files.

CSS Injection and $(document).ready()

According to the jQuery documentation, external CSS files should precede scripts that leverage the ready() function. This ensures that element properties are adequately defined before jQuery execution commences. If this is not observed, sporadic issues, particularly in WebKit browsers, may manifest.

Browser Behavior: CSS Loading and JS Execution

Although CSS is loaded asynchronously, you might anticipate that JS execution would commence and conclude before CSS completion. However, empirical testing reveals that in particular browsers, such as Internet Explorer, sometimes JS execution is deferred until after CSS loading. This is despite the waterfall indicating that JS loading precedes CSS download.

CSS Placement and Execution Timing

Placing external CSS below all JS scripts can accelerate the execution of the ready() function. Nevertheless, tests demonstrate that this approach does not consistently guarantee the delayed execution of JS until CSS is fully loaded. This behavior seems to hinge upon whether external CSS and JS are utilized.

Conclusion

While it is prudent to follow the advice of the jQuery documentation and include CSS prior to JS scripts, it is essential to be aware of the nuanced browser behaviors surrounding CSS loading and JS execution. By understanding these complexities, you can proactively address potential issues and optimize your scripts for reliable and efficient execution.

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