Does $(document).ready() Guarantee CSS Injection into the DOM?

$(document).ready() and CSS DOM Injection

When executing scripts using $(document).ready(), there may be concerns about whether all CSS values have been injected into the DOM at that time. Here's an in-depth analysis:

DOM-ready vs. CSS-ready

In the past, $(document).ready() attempted to ensure that all CSS stylesheets were loaded before executing scripts. However, starting with release 1.3, jQuery no longer makes such guarantees. The recommendation is to include all CSS files before scripts on the page to avoid sporadic issues with element properties not being correctly defined.

CSS Rendering and $(document).ready()

jQuery ready() does not wait for CSS to render; hence, you may still see visual changes when it kicks in, even if CSS is not fully loaded.

Why JavaScript Delays When CSS is Delayed

An unexpected behavior some developers have observed is that JavaScript execution can be delayed even after the CSS file has started loading. This may not seem logical since CSS is loaded asynchronously and should not affect JavaScript loading. However, testing has shown that in some cases, specific combinations of external JavaScript and CSS loading can lead to such delays.

Solution: CSS before JavaScript

To prevent potential issues, the best practice remains to include CSS files above all JavaScript code. While it may not always prevent JavaScript delays, it avoids problems with element properties not being correctly defined.

Additional Considerations

• External resources, such as scripts and images, can impact loading behavior.
• Different browsers handle CSS and JavaScript loading slightly differently.
• Some browser extensions or third-party scripts may also interfere with the expected loading sequence.

Conclusion

By following the recommendations to include CSS before JavaScript, you can minimize the risk of problems related to CSS injection when using $(document).ready(). If you encounter unexpected delays, thorough testing and careful consideration of the above factors can help identify the underlying cause.

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