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In countless places online I have seen the recommendation to include CSS prior to JavaScript. The reasoning is generally, of this form:
When it comes to ordering your CSS and JavaScript, you want your CSS to come first. The reason is that the rendering thread has all the style information it needs to render the page. If the JavaScript includes come first, the JavaScript engine has to parse it all before continuing on to the next set of resources. This means the rendering thread can't completely show the page, since it doesn't have all the styles it needs.
My actual testing reveals something quite different:
I use the following Ruby script to generate specific delays for various resources:
require 'rubygems' require 'eventmachine' require 'evma_httpserver' require 'date' class Handler < EventMachine::Connection include EventMachine::HttpServer def process_http_request resp = EventMachine::DelegatedHttpResponse.new( self ) return unless @http_query_string path = @http_path_info array = @http_query_string.split("&").map{|s| s.split("=")}.flatten parsed = Hash[*array] delay = parsed["delay"].to_i / 1000.0 jsdelay = parsed["jsdelay"].to_i delay = 5 if (delay > 5) jsdelay = 5000 if (jsdelay > 5000) delay = 0 if (delay < 0) jsdelay = 0 if (jsdelay < 0) # Block which fulfills the request operation = proc do sleep delay if path.match(/.js$/) resp.status = 200 resp.headers["Content-Type"] = "text/javascript" resp.content = "(function(){ var start = new Date(); while(new Date() - start < #{jsdelay}){} })();" end if path.match(/.css$/) resp.status = 200 resp.headers["Content-Type"] = "text/css" resp.content = "body {font-size: 50px;}" end end # Callback block to execute once the request is fulfilled callback = proc do |res| resp.send_response end # Let the thread pool (20 Ruby threads) handle request EM.defer(operation, callback) end end EventMachine::run { EventMachine::start_server("0.0.0.0", 8081, Handler) puts "Listening..." } The above mini server allows me to set arbitrary delays for JavaScript files (both server and client) and arbitrary CSS delays. For example, http://10.0.0.50:8081/test.css?delay=500 gives me a 500 ms delay transferring the CSS.
I use the following page to test.
<!DOCTYPE html> <html> <head> <title>test</title> <script type='text/javascript'> var startTime = new Date(); </script> <link href="http://10.0.0.50:8081/test.css?delay=500" type="text/css" rel="stylesheet"> <script type="text/javascript" src="http://10.0.0.50:8081/test2.js?delay=400&jsdelay=1000"></script> </head> <body> <p> Elapsed time is: <script type='text/javascript'> document.write(new Date() - startTime); </script> </p> </body> </html> When I include the CSS first, the page takes 1.5 seconds to render:
When I include the JavaScript first, the page takes 1.4 seconds to render:
I get similar results in Chrome, Firefox and Internet Explorer. In Opera however, the ordering simply does not matter.
What appears to be happening is that the JavaScript interpreter refuses to start until all the CSS is downloaded. So, it seems that having JavaScript includes first is more efficient as the JavaScript thread gets more run time.
Am I missing something, is the recommendation to place CSS includes prior to JavaScript includes not correct?
It is clear that we could add async or use setTimeout to free up the render thread or put the JavaScript code in the footer, or use a JavaScript loader. The point here is about ordering of essential JavaScript bits and CSS bits in the head.
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You should learn HTML and CSS before JavaScript. HTML and CSS are the core technologies for building web pages and applications. HTML defines the structure of your content, CSS determines the style and layout, and JS makes the content interactive; therefore, you should learn them in that order.
When it comes to ordering your CSS and JavaScript, you want your CSS to come first. The reason is that the rendering thread has all the style information it needs to render the page. If the JavaScript includes come first, the JavaScript engine has to parse it all before continuing on to the next set of resources.
Cascading Stylesheets — or CSS — is the first technology you should start learning after HTML. While HTML is used to define the structure and semantics of your content, CSS is used to style it and lay it out.
The typical answer is: Add JavaScript code by the end of the </body> tag and. Add CSS code in-between the <head> tags.
This is a very interesting question. I've always put my CSS <link href="...">s before my JS <script src="...">s because "I read one time that it's better." So, you're right; it's high time we do some actual research!
I set up my own test harness in Node (code below). Basically, I:
<head> to execute<body> to execute, which is analogous to DOMReady.First, with the CSS file delayed by 500ms:
Browser: Chrome 18 | IE 9 | Firefox 9 CSS: first last | first last | first last ======================================================= Header Exec | | | Average | 583ms 36ms | 559ms 42ms | 565ms 49ms St Dev | 15ms 12ms | 9ms 7ms | 13ms 6ms ------------|--------------|--------------|------------ Body Exec | | | Average | 584ms 521ms | 559ms 513ms | 565ms 519ms St Dev | 15ms 9ms | 9ms 5ms | 13ms 7ms Next, I set jQuery to delay by 500ms instead of the CSS:
Browser: Chrome 18 | IE 9 | Firefox 9 CSS: first last | first last | first last ======================================================= Header Exec | | | Average | 597ms 556ms | 562ms 559ms | 564ms 564ms St Dev | 14ms 12ms | 11ms 7ms | 8ms 8ms ------------|--------------|--------------|------------ Body Exec | | | Average | 598ms 557ms | 563ms 560ms | 564ms 565ms St Dev | 14ms 12ms | 10ms 7ms | 8ms 8ms Finally, I set both jQuery and the CSS to delay by 500ms:
Browser: Chrome 18 | IE 9 | Firefox 9 CSS: first last | first last | first last ======================================================= Header Exec | | | Average | 620ms 560ms | 577ms 577ms | 571ms 567ms St Dev | 16ms 11ms | 19ms 9ms | 9ms 10ms ------------|--------------|--------------|------------ Body Exec | | | Average | 623ms 561ms | 578ms 580ms | 571ms 568ms St Dev | 18ms 11ms | 19ms 9ms | 9ms 10ms First, it's important to note that I'm operating under the assumption that you have scripts located in the <head> of your document (as opposed to the end of the <body>). There are various arguments regarding why you might link to your scripts in the <head> versus the end of the document, but that's outside the scope of this answer. This is strictly about whether <script>s should go before <link>s in the <head>.
In modern DESKTOP browsers, it looks like linking to CSS first never provides a performance gain. Putting CSS after script gets you a trivial amount of gain when both CSS and script are delayed, but gives you large gains when CSS is delayed. (Shown by the last columns in the first set of results.)
Given that linking to CSS last does not seem to hurt performance but can provide gains under certain circumstances, you should link to external stylesheets after you link to external scripts only on desktop browsers if the performance of old browsers is not a concern. Read on for the mobile situation.
Historically, when a browser encountered a <script> tag pointing to an external resource, the browser would stop parsing the HTML, retrieve the script, execute it, then continue parsing the HTML. In contrast, if the browser encountered a <link> for an external stylesheet, it would continue parsing the HTML while it fetched the CSS file (in parallel).
Hence, the widely-repeated advice to put stylesheets first – they would download first, and the first script to download could be loaded in parallel.
However, modern browsers (including all of the browsers I tested with above) have implemented speculative parsing, where the browser "looks ahead" in the HTML and begins downloading resources before scripts download and execute.
In old browsers without speculative parsing, putting scripts first will affect performance since they will not download in parallel.
Speculative parsing was first implemented in: (along with the percentage of worldwide desktop browser users using this version or greater as of Jan 2012)
In total, roughly 85% of desktop browsers in use today support speculative loading. Putting scripts before CSS will have a performance penalty on 15% of users globally; YMMV based on your site's specific audience. (And remember that number is shrinking.)
On mobile browsers, it's a little harder to get definitive numbers simply due to how heterogeneous the mobile browser and OS landscape is. Since speculative rendering was implemented in WebKit 525 (released Mar 2008), and just about every worthwhile mobile browser is based on WebKit, we can conclude that "most" mobile browsers should support it. According to quirksmode, iOS 2.2/Android 1.0 use WebKit 525. I have no idea what Windows Phone looks like.
However, I ran the test on my Android 4 device, and while I saw numbers similar to the desktop results, I hooked it up to the fantastic new remote debugger in Chrome for Android, and Network tab showed that the browser was actually waiting to download the CSS until the JavaScripts completely loaded – in other words, even the newest version of WebKit for Android does not appear to support speculative parsing. I suspect it might be turned off due to the CPU, memory, and/or network constraints inherent to mobile devices.
Forgive the sloppiness – this was Q&D.
app.js
var express = require('express') , app = express.createServer() , fs = require('fs'); app.listen(90); var file={}; fs.readdirSync('.').forEach(function(f) { console.log(f) file[f] = fs.readFileSync(f); if (f != 'jquery.js' && f != 'style.css') app.get('/' + f, function(req,res) { res.contentType(f); res.send(file[f]); }); }); app.get('/jquery.js', function(req,res) { setTimeout(function() { res.contentType('text/javascript'); res.send(file['jquery.js']); }, 500); }); app.get('/style.css', function(req,res) { setTimeout(function() { res.contentType('text/css'); res.send(file['style.css']); }, 500); }); var headresults={ css: [], js: [] }, bodyresults={ css: [], js: [] } app.post('/result/:type/:time/:exec', function(req,res) { headresults[req.params.type].push(parseInt(req.params.time, 10)); bodyresults[req.params.type].push(parseInt(req.params.exec, 10)); res.end(); }); app.get('/result/:type', function(req,res) { var o = ''; headresults[req.params.type].forEach(function(i) { o+='\n' + i; }); o+='\n'; bodyresults[req.params.type].forEach(function(i) { o+='\n' + i; }); res.send(o); }); css.html
<!DOCTYPE html> <html> <head> <title>CSS first</title> <script>var start = Date.now();</script> <link rel="stylesheet" href="style.css"> <script src="jquery.js"></script> <script src="test.js"></script> </head> <body> <script>document.write(jsload - start);bodyexec=Date.now()</script> </body> </html> js.html
<!DOCTYPE html> <html> <head> <title>CSS first</title> <script>var start = Date.now();</script> <script src="jquery.js"></script> <script src="test.js"></script> <link rel="stylesheet" href="style.css"> </head> <body> <script>document.write(jsload - start);bodyexec=Date.now()</script> </body> </html> test.js
var jsload = Date.now(); $(function() { $.post('/result' + location.pathname.replace('.html','') + '/' + (jsload - start) + '/' + (bodyexec - start)); }); jquery.js was jquery-1.7.1.min.js
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There are two main reasons to put CSS before JavaScript.
Old browsers (Internet Explorer 6-7, Firefox 2, etc.) would block all subsequent downloads when they started downloading a script. So if you have a.js followed by b.css they get downloaded sequentially: first a then b. If you have b.css followed by a.js they get downloaded in parallel so the page loads more quickly.
Nothing is rendered until all stylesheets are downloaded - this is true in all browsers. Scripts are different - they block rendering of all DOM elements that are below the script tag in the page. If you put your scripts in the HEAD then it means the entire page is blocked from rendering until all stylesheets and all scripts are downloaded. While it makes sense to block all rendering for stylesheets (so you get the correct styling the first time and avoid the flash of unstyled content FOUC), it doesn't make sense to block rendering of the entire page for scripts. Often scripts don't affect any DOM elements or just a portion of DOM elements. It's best to load scripts as low in the page as possible, or even better load them asynchronously.
It's fun to create examples with Cuzillion. For example, this page has a script in the HEAD so the entire page is blank until it's done downloading. However, if we move the script to the end of the BODY block the page header renders since those DOM elements occur above the SCRIPT tag, as you can see on this page.
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