Progressive Downloads and Rendering - take #2

Progressive downloads
and rendering
Stoyan Stefanov, Yahoo!
HighLoad++, Moscow, 2010
http://slideshare.net/stoyan/

Importance of performance
•  Psychology, physiology
•  Effects of waiting
•  “Time is money”
•  Make people happy

Time is relative
•  Sometimes crawls
•  Sometimes flies
•  “it depends”

Durations
actual
expected
perceived
rem’d
time

It feels slower when…
•  Unpleasant
•  Unknown
•  Boring
•  Too much to keep track

First time experience
•  Unfamiliar = slow
•  Optimize empty cache or
there will be no full cache

The basics
•  Reducing the # HTTP
•  Gzip
•  Minification
•  Image smushing
•  Expires
•  CDN

The basics
•  Yahoo!’s best practices +
YSlow
http://developer.yahoo.com/performance/
•  Google’s too +
Page Speed
http://code.google.com/speed/

Agenda
1.  Prevent download blocks:
scripts, styles, CC, favicon
2.  Ways to render sooner:
flush, data URIs, lazy loading,
lazy evaluation, preloading,
animations

JavaScript blocks
html
js
png
png

JavaScript blocks
•  A no-no!
<script src="jquery.js"></script>
<script src="jquery.twitter.js"></script>
<script src="jquery.cookie.js"></script>
<script src="myapp.js"></script>

This waterfall looks ridiculous
html
js
png
png
js
js
js

JavaScript at the bottom
html
js
png
png

Non-blocking JavaScript
•  defer and async
•  Defer: IE innovation, ok to
delay, but keep order
•  Async: HTML5, whatever
<script async src="my.js" onload="doIt()"></script>
<script defer src="my.js" onload="doIt()"></script>

defer and async timeline
DOMContentLoaded load
async
defer

•  Asynchronous loading
var h, js = document.createElement('script');
js.src = 'myscript.js';
h = document.getElementsByTagName('head')[0];
h.appendChild(js);
html
js
png
png

•  Others:
- iframe
- XHR
- <object>
- …

CSS blocks rendering
•  The worst component type
•  Place way at the top
•  @media print, etc in the
same external CSS
http://www.phpied.com/delay-loading-your-print-css/
http://www.phpied.com/rendering-styles/

CSS block
downloads?
But they do block:
•  When followed
by an inline script
•  When in conditional
comments

Inline CSS
•  Google search
•  Bing.com: inline + postload

Same domain
•  If you split across domains
•  and if you don’t use CDN
•  Saves a DNS lookup
•  e.g. Google and Bing’s CDN

With conditionally commented
CSS file
http://www.phpied.com/ 
conditional-comments-block-downloads/

What…?! Case #1
  <link type="text/css" rel="stylesheet"
        href="1.css">
  <!‐‐[if IE 6]>
    <link type="text/css" rel="stylesheet"
          href="ie.css">
  <![endif]‐‐>

What…?! Case #2
<!‐‐[if IE 6]>
<body class="ie6">
<![endif]‐‐>
<!‐‐[if !IE]><!‐‐>
<body>
<!‐‐<![endif]‐‐>

Solution for case #1
<!DOCTYPE html>
<!‐‐[if IE 6]><![endif]‐‐>
<html>
...

Solution for case #2
<!‐‐[if IE 6]>
<html class="ie6">
<![endif]‐‐>
<!‐‐[if !IE]><!‐‐>
<html>
<!‐‐<![endif]‐‐>

flush() early
html
png
js
css
html
png
js
css
✔


flush()
<html>
<head>
<script src="my.js"
type="text/javascript"></script>
<link href="my.css"
type="text/css" rel="stylesheet" />
</head>
<body>
....
<?php flush() ?>

Chunked encoding
HTTP/1.1 200 OK
Content‐Type: text/plain
Transfer‐Encoding: chunked
25
This is the data in the first chunk
1C
and this is the second one
0

Chunked encoding
•  Progressive rendering
- Semantic app chunks
vs.
- Server-level chunks

Progressive rendering
Chunk
#1
Chunk
#2
Chunk
#3

<!doctype html>
<html>
<head><title>My App</title></head>
<body>
<div id="header">
<img src="logo.png" />
...
</div> 
... The full body of the page ...

<script src="all_20100925.js"></script>
</body>
</html>

Progressive + source order
1
2
3
4

HTTP chunking: not only HTML
•  Google Instant
•  /*""*/ - delimited JSON
pieces
•  Chunk #1 suggestions
•  Chunk #2 results
http://tinyurl.com/chunkview

Fewer HTTP requests
•  Inline images:
in CSS sprites
with data: URI scheme
http://csssprites.com
http://spriteme.org

Fewer HTTP requests
•  data: URI scheme
$ php ‐r "echo base64_encode(file_get_contents('my.png'));”
iVBORw0KGgoAAAANSUhEUgAAAAQAAAADCAIAAAA7ljmRAAAAGElEQVQIW2P4
DwcMDAxAfBvMAhEQMYgcACEHG8ELxtbPAAAAAElFTkSuQmCC

Fewer HTTP requests
background‐image: url("data:image/png;base64,iVBORw0KG...");

Fewer HTTP requests
<img src="data:image/png;base64,iVBOR..." />

Both
•  flushes
•  data: URIs

Fewer HTTP requests
•  works in IE!...

Fewer HTTP requests
•  works in IE8!

Fewer HTTP requests
•  MHTML for IE < 8

MHTML
•  MIME HTML
•  Works in IE 6,7
•  Indeed it actually absolutely
does work in IE7/Vista too
http://phpied.com 
/the-proper-mhtml-syntax/

MHTML - one part
Content-Location: myimage
Content-Transfer-Encoding: base64
iVBORw0KGgoAAAANSU....U5ErkJggg==

MHTML - multi parts
Content-Type: multipart/related; boundary="MYSEPARATOR"
--MYSEPARATOR
[here comes part one]
--MYSEPARATOR
[here's part two]
--MYSEPARATOR--
The
double-
dash of
doom

MHTML.css – all together
/*
Content-Type: multipart/related; boundary="MYSEPARATOR"
--MYSEPARATOR
iVBORw0KGgoAAAANSUhEUgAAABgAAAAYCAMAAADXqc3KAAAD....U5ErkJggg==
--MYSEPARATOR
Content-Location: another
iVBORw0KGgoAAAANSUhEUgAAABgAAAAYCAMAAADXqc3KAAAA....U5ErkJggg==
--MYSEPARATOR--
*/
.myclass {
background-image:url(mhtml:http://example.org/styles.css!myimage);
}
.myotherclass {
background-image:url(mhtml:http://example.org/styles.css!another);
}

MHTML: inline too<!doctype html>
<html>
<head>
<title>Look Ma' No HTTP requests</title>
<style type="text/css">
/*
Content-Type: multipart/related; boundary="_"
--_
Content-Location:locoloco
Content-Transfer-Encoding:base64
iVBOR...CC
--_
Content-Location:polloloco
iVBOR....gg==
--_--
*/
.image1 {
background-image: url("data:image/png;base64,iVBOR...CC"); /* normal */
*background-image: url(mhtml:http://...html!locoloco); /* IE < 8 */
}
.image2 {
background-image: url("data:image/png;base64,iVBOR...gg=="); /* normal */
*background-image: url(mhtml:http://...html!polloloco); /* IE < 8 */
}
body {
font: bold 24px Arial;
}
</style>
</head>
<body>
<h1>MHTML + Data:URIs inline in <code>style</code></h1>
<p class="image1">hello<br>hello</p>
<p class="image2">bonjour<br>bonjour</p>
</body>
</html>
http://phpied.com 
/inline-mhtml-data-uris/

<!doctype html>
<html>
<head>
<title>Look Ma' No HTTP requests</title>
<style type="text/css">
...

/*
Content-Type: multipart/related; boundary="_"
--_
Content-Location:locoloco
iVBOR...CC
--_
Content-Location:polloloco
iVBOR....gg==
--_--
*/

.image1 {
background-image: url("data:image/png;base64,iVBOR...CC"); /* normal */
*background-image: url(mhtml:http://...html!locoloco); /* IE < 8 */
}
.image2 {
background-image: url("data:image/png;base64,iVBOR...gg=="); /* normal */
*background-image: url(mhtml:http://...html!polloloco); /* IE < 8 */
}
body {
font: bold 24px Arial;
}

...
</style>
</head>
<body>
<h1>MHTML + Data:URIs inline in <code>style</code></h1>
<p class="image1">hello<br>hello</p>
<p class="image2">bonjour<br>bonjour</p>
</body>
</html>

MHTML + data URI
•  X-browser
single request
web apps

Single request
•  WT☠?
•  Separation of concerns
•  Content-presentation-
behavior
•  yes, it’s a tradeoff

MHTML + data URI
•  drawback: repeats the same
encoded image
•  solutions:
- browser-specific CSS
- keep close = better gzip
- or… an ingenious hack

Single stream MHTML/data URI
•  image header + css + data
/9j/4AA0;background-image:url(data:image/jpeg;base64;00,/9j/4AA0
Reality:
IE:
Others:
http://habrahabr.ru/blogs/webdev/90761/

/*
Content-Type: multipart/related; boundary="granitza"
--granitza
Content-Type: text/css;
*/
#myid {
/*
--granitza
Content-Type: image/jpeg;*/
/9j/4AA0;background-image:url(data:image/jpeg;base64;00,/9j/4AAQSkZJRgA...);
/*
--granitza
*/
background-image: url(mhtml:http://localhost/my.css!myimage) !ie;
}
/*
--granitza--
*/

/*
--granitza
*/
#myid {
/*
--granitza
/*
--granitza
*/
}
/*
--granitza--
*/
???

/*
--granitza
*/
#myid {
/*
--granitza
/*
--granitza
*/
}
/*
--granitza--
*/
???1
2
3

Single stream MHTML/data URI
•  And for <img> too!

Single stream <img>
<h2>Hello Kitty</h2>
<!
--granitza
Content-Type: image/gif
2 invalid MHTML headers lines followed by data--><![if gt IE 7]>
<img width="16" height="16" src="data:image/gif;base64,
R0lGODl ... FxMKVvDijNQaodOl+N5Pk+pmIX7brGweCg4SCEhEAOw=="><![endif]>

<p> more page... </p>

Lazy loading aka post-loading
•  After-onload
•  Some images
•  Below the fold (on scroll)
•  Hidden content e.g. tabs

Amazon’s lazy bestsellers
•  Page’s purpose is ranking
•  Details can come later
•  via onload XHR
•  JS off = no details
•  but that’s fine (see bullet #1)

GMail mobile’s lazy JS
<!doctype html>
<html><body>
...
<script id="lazy">/*
console.log("I can wait");
*/</script>
...
<script>
console.log("I'm needed");
window.onload = function () {
var comment = document.getElementById('lazy')
.innerHTML,
code = comment.substring(3, comment.length - 3);
eval(code);
};
</script>
</body></html>
http://googlecode.blogspot.com/2009/09 
/gmail-for-mobile-html5-series-reducing.html

Lazy HTML
<!doctype html>
<html><body>
...
<div id="lazy"></div>
...
<script>
window.onload = function () {
var el = document.getElementById('lazy'),
inner = el.innerHTML,
code = inner.substring(4, inner.length - 3);
el.innerHTML = code;
};
</script>
</body></html>
http://phpied.com... (coming-soon)

Lazy HTML test
•  500K (200K gzipped) HTML doc
•  “Sherlock Holmes”
•  comment out 95%
•  still one whole chapter left
http://www.phpied.com/files/lazyhtml/start.html

Lazy HTML - misc
•  Who loads a book?
•  Use case: blog comments
•  SEO? Content is hidden
•  What about display: none?
•  The test page was simple-to-
render, no complex layout

Preloads
•  Anticipate next page
•  Problems:
- does next page anticipate you?
- parsing and execution time
•  <link prefetch="http://..">

Preload sans execute
var preload;
if (/*@cc_on!@*/false) { // IE
preload = function (file) {
new Image().src = file;
};
} else {
preload = function (file) {
var obj = document.createElement('object'),
body = document.body;
obj.width = 0;
obj.height = 0;
obj.data = file;
body.appendChild(obj);
};
}

Preload, then execute
var loader = function (file, callback) {
var obj = document.createElement('object'),
body = document.body;
obj.width = 0;
obj.height = 0;
obj.data = file;
obj.onload = function () {
var h, js = document.createElement('script');
js.src = file;
js.onload = callback;
h = document.getElementsByTagName('head')[0];
h.appendChild(js);
};
body.appendChild(obj);
};

Chrome search
•  In-browser search box
•  Gives suggestions as you type
•  Visual suggestions in IE8+

IE8 Visual
Search Suggestions
...
<Item>
<Text>Currently: Partly Cloudy, 67F</Text>
<Description>High: 71F Low: 63F</Description>
<Url>http://weather.yahoo.com/forecast/
USCA1024_f.html</Url>
<Image source="http://l.yimg.com/a/i/us/we/31/30.gif"
alt="Partly Cloudy" width="31" height="31"/>
</Item>
...

IE8 Visual Search Preload
...
<Item>
<Text>any search suggestion</Text>
<Image
source="http://path/to/sprite.png"
width="0"
height="0"/>
</Item>
...

IE8 Visual Search Preload
•  didn’t work for CSS and JS

IE8 Visual Search
•  preload images, e.g. sprite
•  DNS lookups via beacons

What not to say…
•  “Everyone is on high-speed
these days”
•  “It’s all in the cache”

Do care about
•  Progressive,
non-blocking,
asynchronous downloads
•  Progressive rendering

Thank you!
Stoyan Stefanov
@stoyanstefanov
http://www.phpied.com
Slides: http://slideshare.net/stoyan/

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