Peterbe.com

Peter Bengtsson's blog

Page 43

Never put external Javascript in the <head>

April 2, 2013
13 comments Web development

First of all, the title is perhaps misleading. Basically, don't put plain script tags that are not async in the head tag.

If you put a piece of javascript in the head of HTML page, the browser will start to download that and proceed down the lines of HTML and download other resources too as it encounters them such as the CSS files.

Then, when all javascript and CSS has been downloaded it will start rendering the page and when it does that it will download any images referenced in the HTML. At roughly the same time it will start to display things on the screen. But it won't do this until the CSS and Javascript has been downloaded.

To repeat: The browser screen will appear blank. It won't start downloading any images if downloading a javascript URL referenced in the head gets stuck.

Here are two perfectly good examples from this morning's routine hunt for news:

Wired.com is guilty

getharvest.com is guilty

Here's what getharvest.com does in their HTML:


<!DOCTYPE html>
<html lang="en">

  <head>
    <script type="text/javascript">var NREUMQ=NREUMQ||[];NREUMQ.push(["mark","firstbyte",new Date().getTime()]);</script>
    <script type="text/javascript" src="http://c761485.r85.cf2.rackcdn.com/gascript.js"></script>
  ...

Why it gets stuck on connecting to c761485.r85.cf2.rackcdn.com I just don't know. But it does. The Internet is like that oftentimes. You simply can't connect to otherwise perfectly configured web servers.

Update-whilst-writing-this-text! As I was writing this text I gave getharvest.com a second chance thinking that most likely the squirrels in my internet tubes will be back up and running to rackcdn.com but then [this happened!/static/cache/bd/02/bd02367be6bbe6d16444051619d88bee.jpg)

So, what's the right thing to do? Simple: don't rely on external resources. For example, can you move the Javascript script tag to the very very bottom of the HTML page. That way it will render as much as it possibly can whilst waiting for the Javascript resource to get unstuck. Or, almost equally you can keep the script tag in the <head> but then but in async attribute on it like this:


<script async type="text/javascript" src="http://c761485.r85.cf2.rackcdn.com/gascript.js"></script>

Another thing you can do is not use an external resource URL (aka. third-party domain). Instead of using cdn.superfast.com/file.js you instead use /file.js. Sure, that fancy CDN might be faster at serving up stuff than your server but looking up a CDN's domain is costing one more DNS lookup which we know can be very expensive for that first-time impression.

I know I'm probably guilty of this new on some of my (now) older projects. For example, if you open aroundtheworldgame.com it won't render anything until it has managed to connect to maps.googleapis.com and dn4avfivo8r6q.cloudfront.net but that's more of an app rather than a web site.

By the way... I wrote some basic code to play around with how this actually works. I decided to put this up in case you want to experiment with it too: https://github.com/peterbe/slowpage

premailer now honours specificity

March 21, 2013
0 comments Python

Thanks to Theo Spears awesome effort premailer now support CSS specificity. What that means is that when linked and inline CSS blocks are transformed into tag style attributes the order is preserved as you'd expect.

When the browser applies CSS to elements it does it in a specific order. For example if you have this CSS:


p.tag { color: blue; }
p { color: red; }

and this HTML:


<p>Regular text</p>
<p class="tag">Special text</p>

the browser knows to draw the first paragraph tag in red and the second paragraph in red. It does that because p.tag is more specific that p.

Before, it would just blindly take each selector and set it as a style tag like this:


<p style="color:red">Regular text</p>
<p style="color:blue; color:red" class="tag">Special text</p>

which is not what you want.

The code in action is here.

Thanks Theo!

HTML whitespace "compression" - don't bother!

March 11, 2013
4 comments Web development

This morning I came across this site on Hacker News. It's a cute site with some basic tips on how to make your sites faster.

It's very much a for-beginners document as all the tips are quite basic. For example it doesn't even mention the use of CDNs.

One tip in particular stood out to me: "it can be useful to minify your HTML with automated tools."
And it links to the htmlcompressor project. Ignore this advice.

What matters 10 times more is Gzip compression. This is usually very easy to set up with Nginx or Apache. It's not something you do in your web framework and if you don't have a web framework, you don't need to manully Gzip HTML files on the filesystem.

For example, downloading the home page here on my blog, at the time of writing, this is: 66,770 bytes big. Hefty, sure, but with all excess whitespace removed it reduces down to 59,356 bytes. But that really doesn't matter when you Gzip.

Gzipped from original version: 18,470 bytes
Gzipped from whitespace trimmed version: 18,086 bytes

The gain is 2% which is definitely not worth the hassle of adding a whitespace compressor.

django-fancy-cache with or without stats

March 11, 2013
1 comment Python, Django

If you use django-fancy-cache you can either run with stats or without. With stats, you can get a number of how many times a cache key "hits" and how many times it "misses". Keeping stats incurs a small performance slowdown. But how much?

I created a simple page that either keeps stats or ignores it. I ran the benchmark over Nginx and Gunicorn with 4 workers. The cache server is a memcached running on the same host (my OSX 10.7 laptop).

With stats:

Average: 768.6 requests/second
Median: 773.5 requests/second
Standard deviation: 14.0

Without stats:

Average: 808.4 requests/second
Median: 816.4 requests/second
Standard deviation: 30.0

That means, roughly that running with stats incurs a 6% slower performance.

The stats is completely useless to your users. The stats tool is purely for your own curiousity and something you can switch on and off easily.

Note: This benchmark assumes that the memcached server is running on the same host as the Nginx and the Gunicorn server. If there was more network in between, obviously all the .incr() commands would cause more performance slowdown.

This site is now 100% inline CSS and no bytes are wasted

March 5, 2013
8 comments Python, Web development, Django

This personal blog site of mine uses django-fancy-cache and mincss.

What that means is that I can cache the whole output of every blog post for weeks and when I do that I can first preprocess the HTML and convert every external CSS into one inline STYLE block which will only reference selectors that are actually used.

To see it in action, right-click and select "View Page Source". You'll see something like this:

/*
Stats about using github.com/peterbe/mincss
-------------------------------------------
Requests:         1 (now: 0)
Before:           81Kb
After:            11Kb
After (minified): 11Kb
Saving:           70Kb
*/
section{display:block}html{font-size:100%;-webkit-text-size-adjust:100%;-ms-tex...

The reason the saving is so huge, in my case, is because I'm using Twitter Bootstrap CSS framework which is awesome but as any framework, it will inevitably contain a bunch of stuff that I don't use. Some stuff I don't use on any page at all. Some stuff is used only on some pages and some other stuff is used only on some other pages.

What I gain by this, is faster page loads. What the browser does is that it, gets a URL, downloads all HTML, opens the HTML to look for referenced CSS (using the link tag) and downloads that too. Once all of that is downloaded, it starts to render the page. Approximately after that it starts to download all referenced Javascript and starts evaluating and executing that.

By not having to download the CSS the browser has one less thing to do. Only one request? Well, that request might be on a CDN (not a great idea actually) so even though it's just 1 request it will involve another DNS look-up.

Here's what the loading of the homepage looks like in Firefox from a US east coast IP.

Granted, a downloaded CSS file can be cached by the browser and used for other pages under the same domain. But, on my blog the bounce rate is about 90%. That doesn't necessarily mean that visitors leave as soon as they arrived, but it does mean that they generally just read one page and then leave. For those 10% of visitors who visit more than one page will have to download the same chunk of CSS more than once. But mind you, it's not always the same chunk of CSS because it's different for different pages. And the amount of CSS that is now in-line only adds about 2-3Kb on the HTML load when sent gzipped.

Getting to this point wasn't easy because I first had to develop mincss and django-fancy-cache and integrate it all. However, what this means is that you can have it done on your site too! All the code is Open Source and it's all Python and Django which are very popular tools.

Welcome to the world django-fancy-cache!

March 1, 2013
3 comments Python, Django

** A Django cache_page on steroids**

Django ships with an awesome view decorator called cache_page which is awesome. But a bit basic too.

What it does is that it stores the whole view response in memcache and the key to it is the URL it was called with including any query string. All you have to do is specify the length of the cache timeout and it just works.
Now, it's got some shortcomings which django-fancy-cache upgrades. These "steroids" are:

Ability to override the key prefix with a callable.
Ability to remember every URL that was cached so you can do invalidation by a URL pattern.
Ability to modify the response before it's stored in the cache.
Ability to ignore certain query string parameters that don't actually affect the view but does yield a different cache key.
Ability to serve from cache but always do one last modification to the response.
Incrementing counter of every hit and miss to satisfy your statistical curiosity needs.

The documentation is here:
https://django-fancy-cache.readthedocs.org/

You can see it in a real world implementation by seeing how it's used on my blog here. You basically use it like this::


from fancy_cache import cache_page

@cache_page(60 * 60)
def myview(request):
    ...
    return render(request, 'template.html', stuff)

What I'm doing with it here on my blog is that I make the full use of caching on each blog post but as soon as a new comment is posted, I wipe the cache by basically creating a new key prefix. That means that pages are never cache stale but the views never have to generate the same content more than once.

I'm also using django-fancy-cache to do some optimizations on the output before it's stored in cache.

mincss version 0.8 is much much faster

February 27, 2013
0 comments Python

Remember mincss from last month? Well, despite it's rather crazy version number has only really had one major release. And it's never really been optimized.

So I took some metrics and was able to find out where all the time is spent. It's basically in this:


for body in bodies:
    for each in CSSSelector(selector)(body):
        return True

That in itself, on its own, is very fast. Just a couple of milliseconds. But the problem was that it happens so god damn often!

So, in version 0.8 it now, by default, first make a list (actually, a set) of every ID and every CLASS name in every node of every HTML document. Then, using this it gingerly tries to avoid having to use CSSSelector(selector) if the selector is quite simple. For example, if the selector is #container form td:last-child and if there is no node with id container then why bother.
It equally applies the same logic to classes.

And now, what you've all been waiting for; the results:

On a big document (20Kb) like my home page...

BEFORE: 4.7 seconds
AFTER: 0.85 seconds

(I ran it a bunch of times and averaged the times which had very little deviation)

So in the first round of optimization it suddenly becomes 500% faster. Pretty cool!

I've made it possible to switch this off just because I haven't yet tested it on equally many sites. All the unit tests pass of course.

mincss now support PhantomJS

February 2, 2013
3 comments Python

Remember mincss from a couple of days ago? Now it supports downloading the HTML, to analyze, using PhantomJS. That's pretty exciting because PhantomJS actually supports Javascript. It's a headless (a web browser without a graphical user interface) Webkit engine. What mincss does is that invokes a simple script like this:


var page = require('webpage').create();
page.open(phantom.args[0], function () {
  console.log(page.content);
  phantom.exit();
});

which will allow any window.onload events to fire which might create more DOM nodes. So, like in this example it'll spit out HTML that contains a <p class="bar"> tag which you otherwise wouldn't get with Python's urllib.urlopen().

The feature was just added (version 0.6.0) and I wouldn't be surprised if there are dragons there because I haven't tried it on a lot of sites. And at the time of writing, I was not able to compile it on my Ubuntu 64bit server so I haven't put it into production yet.

Anyway, with this you can hopefully sprinkle less of those /* no mincss */ comments into you CSS.

mincss in action - sample report from the wild

January 22, 2013
10 comments Python, Web development

First of all, to find out what mincss is read this blog post which explains what the heck this new Python tool is.

My personal website is an ideal candidate for using mincss because it uses an un-customized Bootstrap CSS which weighs over 80Kb (minified) and on every page hit, the rendered HTML is served directly from memcache so dynamic slowness is not a problem. With that, what I can do is run mincss just before the rendered (from Django) output HTML is stored in memcache. Also, what I can do is take ALL inline style blocks and all link tags and combine them into one big inline style block. That means that I can reduce any additional HTTP connections needed down to zero! Remember, "Minimize HTTP Requests" is the number one web performance optimization rule.

To get a preview of that, compare https://www.peterbe.com/about with https://www.peterbe.com/about3. Visually no difference. But view the source :)

Before:

After:

Voila! One HTTP request less and 74Kb less!

Now, as if that wasn't good enough, let's now take into account that the browser won't start rendering the page until the HTML and ALL CSS is "downloaded" and parsed. Without further ado, let's look at how much faster this is now:

Before:

report

After:

report

How cool is that! The "Start Render" event is fired after 0.4 seconds instead of 2 seconds!

Note how the "Content Download" isn't really changing. That's because no matter what the CSS is, there's still a tonne of images yet to download.

That example page is interesting too because it contains a piece of Javascript that is fired on the window.onload that creates little permalink links into the document and the CSS it needs is protected thanks to the /* no mincss */ trick as you can see here.

The code that actually implements mincss here is still very rough and is going to need some more polishing up until I publish it further.

Anyway, I'm really pleased with the results. I'm going to tune the implementation a bit further and eventually apply this to all pages here on my blog. Yes, I understand that the CSS, if implemented as a link, can be reused thanks to the browser's cache but visitors of my site rarely check out more than one page. In fact, the number of "pages per visit" on my blog is 1.17 according to Google Analytics. Even if this number was bigger I still think it would be a significant web performance boost.

UPDATE

Steve Souders points out a flaw in the test. See his full comment below. Basically, what appears to happen in the first report, IE8 downlads the file c98c3dfc8525.css twice even though it returns as a 200 the first time. No wonder that delays the "Start Render" time.

So, I re-ran the test with Firefox instead (still from the US East coast):

Before:

report

After:

report

That still shows a performance boost from 1.4 seconds down to 0.6 seconds when run using Firefox.

Perhaps it's a bug in Webpagetest or perhaps it's simply how IE8 works. In a sense it "simulates" the advantages of reducing the dependency on extra HTTP requests.

mincss "Clears the junk out of your CSS"

January 21, 2013
30 comments Python, Web development

A project I started before Christmas (i.e. about a month ago) is now production ready.

mincss (code on github) is a tool that when given a URL (or multiple URLs) downloads that page and all its CSS and compares each and every selector in the CSS and finds out which ones aren't used. The outcome is a copy of the original CSS but with the selectors not found in the document(s) removed. It goes something like this:


>>> from mincss.processor import Processor
>>> p = Processor()
>>> p.process_url('https://www.peterbe.com')
>>> p.process()
>>> p.inlines
[]
>>> p.links
[<mincss.processor.LinkResult object at 0x10a3bbe50>, <mincss.processor.LinkResult object at 0x10a4d4e90>]
>>> one = p.links[0]
>>> one.href
'//d1ac1bzf3lrf3c.cloudfront.net/static/CACHE/css/c98c3dfc8525.css'
>>> len(one.before)
83108
>>> len(one.after)
10062
>>> one.after[:70]
u'header {display:block}html{font-size:100%;-webkit-text-size-adjust:100'

To whet your appetite, running it on any one of my pages here on my blog it goes from: 82Kb down to 7Kb. Before you say anything; yes I know its because I using a massive (uncustomized) Twitter Bootstrap file that contains all sorts of useful CSS that I'm not using more than 10% of. And yes, those 10% on one page might be different from the 10% on another page and between them it's something like 15%. Add a third page and it's 20% etc. But, because I'm just doing one page at a time, I can be certain it will be enough.

One way of using mincss is to run it on the command line and look at the ouput, then audit it and give yourself an idea of selectors that aren't used. A safer way is to just do one page at a time. It's safer.

The way it works is that it parses the CSS payload (from inline blocks or link tags) with a relatively advanced regular expression and then loops over each selector one at a time and runs it with cssselect (which uses lxml) to see if the selector is used anywhere. If the selector isn't used the selector is removed.

I know I'm not explaining it well so I put together a little example implementation which you can download and run locally just to see how it works.

Now, regarding Javascript and DOM manipulations and stuff; there's not a lot you can do about that. If you know exactly what your Javascript does, for example, creating a div with class loggedin-footer you can prepare your CSS to tell mincss to leave it alone by adding /* no mincss */ somewhere in the block. Again, look at the example implementation for how this can work.

An alternative is to instead of using urllib.urlopen() you could use a headless browser like PhantomJS which will run it with some Javascript rendering but you'll never cover all bases. For example, your page might have something like this:


$(function() {
  $.getJSON('/is-logged-in', function(res) {
    if (res.logged_in) {
      $('<div class="loggedin-footer">').appendTo($('#footer'));
    }
  });
});

But let's not focus on what it can not do.

I think this can be a great tool for all of us who either just download a bloated CSS framework or you have a legacy CSS that hasn't been updated as new HTML is added and removed.

The code is Open Source (of course) and patiently awaiting your pull requests. There's almost full test coverage and there's still work to be done to improve the code such as finding more bugs and optimizing.

Also, there's a rough proxy server you can start that attempts to run it on any URL. You start it like this:

pip install Flask
cd mincss/proxy
python app.py

and then you just visit something like http://localhost:5000/www.peterbe.com/about and you can see it in action. That script needs some love since it's using lxml to render the processed output which does weird things to some DOM elements.

I hope it's of use to you.

UPDATE

Published a blog post about using mincss in action

UPDATE 2

cssmin now supports downloading using PhantomJS which means that Javascript rendering will work. See this announcement

UPDATE 3

Version 0.8 is 500% faster now for large documents. Make sure you upgrade!