Prioritize your critical css and images to render your site fast velocity nyc 2013

Prioritize Your Critical CSS and Images

WebRTCfast
to render your site
Jan-Willem Maessen

PageSpeed Optimization Library
Google
jmaessen@google.com

Prioritizing Critical CSS and Images Makes Render Fast

jmaessen@google

Inline critical CSS in <head>
Inline low res visible images
Lazy load everything else
jmaessen@google

Users Expect Speed
Delay

User reaction

0 - 100 ms

Instant

100 - 300 ms

Slight perceptible delay

300 - 1000 ms

Task focus, perceptible delay

1 s+

Mental context switch

10 s+

I'll come back later...

Our pages
should render in
1000 ms or less
Website Abandonment (Strangeloop)

Ilya Grigorik (Go to his talk)

Mobile web performance is
crippled by network latency
We need to build fast pages for today’s devices

jmaessen@google

Network Performance Varies Even in the Lab

jmaessen@google

Optimistic But Useful Model of HTTP Connection
What is Happening

Time

Total data

(200 ms RTT)

(KB)

Control plane (200-2500 ms)

–

DNS lookup

200 ms

–

TCP Connection

400 ms

–

First 10 packets (15K)

600 ms

14K

Next 20 packets (29K)

800 ms


1000 ms

If your site is reached via a mobile
redirect, you’re already down here.
Time

Total data

(200 ms RTT)

(KB)

TCP DNS lookup
Connection

800 ms

–

44K

First 10 packets (15K)
TCP Connection

1000 ms

14K
–

88K

First 10
(15K)

1200 ms

44K
14K

40
(44K)

1400 ms

88K
44K


1600 ms

88K

Actually, all these numbers exceed the
384Kbit/s (48K/s) bandwidth of old 3G
networks.

Resource request
(original host)
(second

jmaessen@google

We don't need to render the entire page...

We need to render above the fold content!

1. One RTT render for above the fold!
2. No redirects + fast server response (<200 ms)
3. Must optimize critical rendering path
a. Inline critical CSS
b. Remove blocking JavaScript
c. Prioritize above-the-fold images

@igrigorik

Why can’t the browser do this?
What CSS will affect the above the fold HTML?
What HTML will be above the fold after styling?
How large is each above the fold image?
Will JavaScript affect the content of the DOM?

jmaessen@google

Step 1: Prioritize Your Critical CSS

jmaessen@google

Let's look at that simple example...
prioritize_critical_css.html
styles/blue.css
<!doctype html>
<html>
<head>
<title>prioritize_critical_css example</title>
<link rel="stylesheet" href="styles/blue.css">
<link rel="stylesheet" href="styles/big.css">
<style>
@import url(styles/all_using_imports.css);
</style>
<link rel="stylesheet" href="
styles/rewrite_css_images.css">
</head>
<body>
<div class="foo" style="display:inline-block;"></div>
Prioritize Critical CSS
…<body>

.blue {color: blue;}
styles/big.css

.big { font-size: 8em; }
.very_large_class… { font-size: 8em; }
…(lots more unused rules)
styles/all_using_imports.css

@import url(yellow.css);
@import url(blue.css);
@import url(bold.css);
styles/rewrite_css_images.css

.foo {
background-image:
url(../images/BikeCrashIcn.png);
width: 100px;
height: 100px;
}

jmaessen@google

What happens when the browser loads our page
styles/blue.css
<!doctype html>
<html>
<head>
<style>
@import url(styles/all_using_imports.css);
</style>
<link rel="stylesheet" href="
styles/rewrite_css_images.css">
</head>
<body>
…<body>

styles/big.css
styles/all_using_imports.css

@import url(yellow.css);
@import url(blue.css);
@import url(bold.css);
styles/rewrite_css_images.css

.foo {
background-image:
First render:
url(../images/BikeCrashIcn.png);
width: 100px;
height: 100px;
}

1.8s

jmaessen@google

Inline the CSS that is actually used on the page
<!doctype html>
<html>
<head>
<style>.blue{color:blue}</style>
<style>.big{font-size:8em}</style>
<style>.blue{color:blue}.bold{font-weight:bold}</style>
<style>.foo{background-image:url(images/BikeCrashIcn.png);
width:100px;height:100px}</style>
</head>
First render:
<body>
…<body>

0.7s

jmaessen@google

Simplest Option: Just Inline All the CSS
● Around 40% of the CSS rules are used in a typical page
● Tweaking a PHP script to inline all the CSS should be easy
● Warning: 44K of compressed CSS on typical page
As always, test and measure – do what the data tells you
for your site.

HTTP Archive data on CSS sizes

jmaessen@google

Identify critical CSS via an Audit

(Manual)

DevTools > Audits > Web Page Performance
Inline the critical styles...
@igrigorik

Let PageSpeed Find Critical CSS Automatically
ModPagespeedEnableFilters prioritize_critical_css

# Apache

pagespeed EnableFilters

# Nginx

prioritize_critical_css;

Instrumented
Page

PageSpeed
Server

Beacon result:
List of critical selectors
Client
Browser

Downside: first access to the page won’t be optimized.
Upside: will be based on actual client browser behavior
jmaessen@google

Rules That Are Triggered by JavaScript Actions
Option 1: Include them in your critical CSS
● Careful: they may not necessarily show up in an audit!
Option 2: Lazyload them
● Load before we run the relevant JavaScript
● Lazyload also works for non-screen styles (eg print)

jmaessen@google

How Not to Lazy Load CSS
<!doctype html>
<html>
<head>
…
</head>
<body>
…
<link
<link
<link
<link

rel="stylesheet"
rel="stylesheet"
rel="stylesheet"
rel="stylesheet"

href="styles/blue.css">
href="styles/big.css">
href="styles/all_using_imports.css">
href="styles/rewrite_css_images.css" media="all">

The browser will notice
the stylesheets when it
reaches the bottom of
the page and block
rendering –
It can’t tell that none of
the rules will apply to
the page.

</body>
</html>
jmaessen@google

How PageSpeed Optimization Library Lazy Loads CSS
<!doctype html>
<html>
<head>
…
</head>
<body>
…
<noscript class="psa_add_styles">
<link rel="stylesheet" href="styles/all_using_imports.css">
<link rel="stylesheet" href="styles/rewrite_css_images.css" media="all">
</noscript>
<script>...Move <noscript> to a div...</script>
</body>
</html>

The noscript block
keeps the browser from
downloading the CSS
until the script runs.
This blocks rendering
when JS is switched off
But the page continues
to work correctly
You can skip this for
non-screen media=
attributes.
jmaessen@google

Step 2: Lazy Load Below the Fold Images

jmaessen@google

Another very simple example...
lazyload_images.html
<!doctype html>
<html><head>...</head>
<body>
* * * * *
<img src="images/BikeCrashIcn.png"> 
* * * * *
* * * * *
* * * * *
* * * * *
* *
Should be below the fold...
* *
<img src="images/Puzzle.jpg"> 
<img src="images/IronChef2.gif"> 
<img src="images/Cuppa.png"> 
</body> </html>

images/BikeCrashIcn.png

images/Puzzle.jpg

images/IronChef2.gif

images/Cuppa.png
jmaessen@google

Lazyloading All the Images
<!doctype html>
<body>
* * * * *
<script>...lazyload code...</script>
<img lazy_src="images/BikeCrashIcn.png" width="100" height="100"> 
* * * * *
* * * * *
* * * * *
* * * * *
* *
* *
<img lazy_src="images/Puzzle.jpg" width="1023" height="766"> 
<img lazy_src="images/IronChef2.gif" width="192" height="256"> 
<img lazy_src="images/Cuppa.png" width="65" height="70"> 
jmaessen@google

Include Dimensions with Lazy Loaded Images

Without image dimensions (left), page reflows on image load
jmaessen@google

Don’t Lazy Load Images Above the Fold!
<!doctype html>
<body>
* * * * *
<img src="images/BikeCrashIcn.png" width="100" height="100"> 
* * * * *
* * * * *
* * * * *
* * * * *
* *
* *

Remember: We’re
trying to get the
browser to fetch
the visible page
content first!

jmaessen@google

Low-Quality Inline Images Above the Fold + Lazy Load
<!doctype html>
<body>
* * * * *
<img lazy_src="images/BikeCrashIcn.png" src="data:image/jpg;..." width="100"
* * * * *
* * * * *
* * * * *
* * * * *
* *
* *

height="100"> 
height="100"> 
height="100"> 
height="100"> 
height="100"> 

jmaessen@google

Effect of our Optimizations: Visual Completeness

Note: Mobile devices should
load all images by OnLoad to
avoid radio restart.

Before: 6.4s
With lazy load: 4.7s

jmaessen@google

Let PageSpeed Find Above the Fold Images
ModPagespeedEnableFilters lazyload_images,insert_image_dimensions
ModPagespeedEnableFilters resize_mobile_images

# Apache


# Nginx

lazyload_images,insert_image_dimensions;
resize_mobile_images;

Instrumented
Page

PageSpeed
Server

Clients vote on what
images are visible
above the fold

Beacon result:
List of visible images
Client
Browser

jmaessen@google

Lazy load your images
Use low-quality inline placeholder
images above the fold
Load all images before radio off
jmaessen@google

A Note About JavaScript
Page Reflows due to JavaScript are a problem
Statically sizing dynamically loaded areas eliminates page jank
Consider loading JS that affects the page earlier
But use asynchronous techniques (eg async ads load)

jmaessen@google

Ideal Page Load Order
Critical CSS (inlined)
Above the fold HTML with inline image previews
Below the fold HTML with image dimensions
Above the fold images
Below the fold images
Non-critical CSS
In parallel
JavaScript*
jmaessen@google

What This Looks Like on a Real Site

jmaessen@google

What This Looks Like on a Real Site
4.4s:
Unopt first render

1.8-1.9s:
First Render
2.2s:
Low Res Banner
2.4s:
Full Res Banner
3.7s:
Dynamic Image
http://www.radiocity.com/tickets.html

jmaessen@google

Thank you!
Slides @

email
mod_pagespeed
nginx_pagespeed
PageSpeed Service

jmaessen@google.com
http://www.modpagespeed.com
http://ngxpagespeed.com
https://developers.google.com/speed/pagespeed/service

Prioritize your critical css and images to render your site fast velocity nyc 2013

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