From the 1.3 release notes:
The ready() method no longer tries to make any guarantees about waiting for all stylesheets to be loaded. Instead all CSS files should be included before the scripts on the page. More Information
From the ready(fn) documentation:
Note: Please make sure that all stylesheets are included before your scripts (especially those that call the ready function). Doing so will make sure that all element properties are correctly defined before jQuery code begins executing. Failure to do this will cause sporadic problems, especially on WebKit-based browsers such as Safari.
Note that the above is not even about actually rendering the CSS, so you may still see the screen change when ready() kicks in. But it should save you from problems.
Actually, I find it a bit strange that just putting the CSS above the JS will solve all issues. The CSS is loaded asynchronously, so JS loading can start and finish while the CSS is still being downloaded. So if the above is a solution, then executing any JS code is then halted until all earlier requests have completed?
I did some testing, and indeed, sometimes JS is delayed until the CSS is loaded. I don’t know why, because the waterfall shows that the JS has completed loading long before downloading the CSS has finished.
See JS Bin for some HTML and its results (this has a 10 second delay), and see webpagetest.org for its waterfall results. This uses some script from Steve Souders’ cuzillion.com to mimic slow responses. In the waterfall, the reference to resource.cgi is the CSS. So, in Internet Explorer, the first external JS starts to load right after the CSS was requested (but that CSS will take another 10 seconds to finish). But the second <script> tag is not executed until the CSS has finished loading as well:
<link rel="stylesheet" type="text/css" href="https://stackoverflow.com/questions/1324568/.a script that delays.cgi" />
<script type="text/javascript" src=".../jquery.min.js"></script>
<script type="text/javascript">
alert("start after the CSS has fully loaded");
$(document).ready(function() {
$("p").addClass("sleepcgi");
alert("ready");
});
</script>
Another test with a second external JS after getting jQuery, shows that the download of the second JS is not started until the CSS has loaded. Here, the first reference to resource.cgi is the CSS, the second the JS:
Moving the stylesheet below all JS indeed shows that the JS (including the ready function) runs much earlier, but even then the jQuery-applied class –which is yet unknown when the JS runs– is used correctly in my quick tests in Safari and Firefox. But it makes sense that things like $(this).height() will yield wrong values at that time.
However, additional testing shows that it is not a generic rule that JS is halted until earlier defined CSS is loaded. There seems to be some combination with using external JS and CSS. I don’t know how this works.
Last notes: as JS Bin includes Google Analytics in each script when running from the bare URL (like jsbin.com/aqeno, the test results are actually changed by JS Bin… It seems that the Output tab on the edit URL such as jsbin.com/aqeno/edit does not include the additional Google Analytics things, and surely yields different results, but that URL is hard to test using webpagetest.org. The reference to Stylesheets Block Downloads in Firefox and JavaScript Execution in IE as given by strager is a good start for a better understanding, but I got many questions left… Also note Steve Souders’ IE8 Parallel Script Loading to make things even more complicated. (The waterfalls above are created using IE7.)
Maybe one should simply believe the release notes and documentation…