CODEWITHSUNDEEP
cssperformancebrowserstylesheet
Luis Abreu
Luis Abreu

Reputation: 4518

Modern browsers, progressive rendering anc CSS at bottom

Currently, I'm investigating several tips/recommendations for improving the performance of web sites. So, I've started with Steve Souders' excellent books (High performance web sites and even faster web sites), but I've got a couple of questions regarding some of the rules that are presented. FOr instance, chapter 5 of High performance web sites say that CSS stylesheets should be put at the top of the page because putting them at the bottom stops the progressive rendering that is performed by the browsers. According to Steve, some browsers (most notably IE) do get stuck with it and show a blank page instead of showing the items progressively. Here's the url for that test page: http://stevesouders.com/hpws/css-bottom.php

Now, I do understand that we're talking about a book with a couple of years and that browsers (including IE) have been updated and improved. The reason I'm asking this is because I can't reproduce the behavior he mentions with any current version of FF, Chrome or IE.

Well, the thing is that Yahoo (http://developer.yahoo.com/performance/rules.html#css_top) and google (https://developers.google.com/speed/docs/best-practices/rendering#PutCSSInHead) still say that.

So, what I'd like to know is if browsers have evolved in this area and this is only problematic for, say IE 8? If that is the case, why haven't yahoo and google updated their recommendations? (btw, I've tried simulating IE7 from within IE11 and still don't see the expected result that is described in the book...)

*UDPATE*One more final note: I've decided to reproduce Steve's cgi script in asp.net and I've created a simple generic handler that does the same thing as the sleep.cgi script. what I'm seeing here is that putting a stylesheet reference (which takes some time to load - I've went with 10 seconds) inside the head ends up producing the blank page problem that is reported in the book. If you put at the end, the browser ends up rendering everything and making a second pass for applying the styles after they have been loaded. In my opinion, this makes sense because when you put the style in the header element, the browser is holding up until it gets the styles before rendering (notice that the other referenced components are still being downloaded on the background, but they're not being shown in the screen). On the other hand, when they're at the bottom, the browser will simply apply the current styles until it gets stuck in the stylesheet. WHen that happens, it will only show the html it has loaded until the stylesheet (if there are any images below it, the browser will still download them but it will only render them after the styles have been loaded).

So, after these tests, I'm starting to think that 1.) I'm missing something here or 2.) yahoo and google recommendations are no longer valid today.

Thoughts?

Thanks guys!

Upvotes: 4

Views: 846

Answers (1)

mz3
mz3

Reputation: 1344

Simply inserting a <link> tag in the footer is not the way to defer stylesheets. The currently accepted method is to attach it using javascript:

<script>
function loadStyleSheet(e){if(document.createStyleSheet)try{document.createStyleSheet(e)}catch(t){}else{var l;l=document.createElement("link"),l.rel="stylesheet",l.type="text/css",l.media="all",l.href=e,document.getElementsByTagName("head")[0].appendChild(l)}}loadStyleSheet("/your/stylesheet.css");
</script>

Optimizing your page for speed involves determining what CSS is above the fold, inlining that part in the header, and loading the main stylesheet later using the above method.

I recommend doing some searches for "above the fold css" and check out Google Pagespeed Insights.

https://developers.google.com/speed/pagespeed/insights/

Upvotes: 2

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