Building high performing web pages

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High Performing web pages
Nilesh Bafna

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How does the web browser renders
a web page?
 Constructing the Object models
 Render tree construction
 Layout stage
 Paint
 Composite
© Perennial Systems Inc
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Constructing Object model
Bytes -> Characters -> Tokens -> Nodes -> Object Model
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Render tree
Render Tree Creation
1) Start at the root node of
the DOM
2) Nodes which are not
visible are omitted like
script, meta etc
3) Additional nodes which
are hidden by CSS are
also omitted from the final
render tree
4) Each visible node and its
CSS rules are merged in
a node
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Layout stage
 This stage involves the calculation of the
exact position and size of every node
within the viewport
 It starts with the top parent and the
output is a box model with exact
absolute pixel positions updated in the
render tree.
<html>
<head>
<meta name="viewport" content="width=device-width,initial-scale=1">
<title>Critial Path: Hello world!</title>
</head>
<body>
<div style="width: 50%">
<div style="width: 50%">Hello world!</div>
</div>
</body>
</html>
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Paint & Composite
 Once the layout is complete the browser issues a Paint setup and
Paint event for converting the render tree to pixels on screen
 Time required to paint varies on the number of nodes and complexity
of the styles. Eg: Solid colors take less time to paint, drop shadows
take more time.
 The drawing happens on different surfaces called layers
 Process of ordering the layers while painting so that they are seen in
the right order on the screen is called Composite.
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How does browser react to scripts
and styles within code?
 While parsing the page, if the browser comes across a script, it has to
stop the creation of the DOM and pass the control to Javascript
engine to execute the script.
 Javascript cannot be executed if any style sheets on the page are still
to be loaded and CSS object model is pending to be formed. Hence,
now the parsing stops till both the CSSOM is created and then the
script is executed. Now consider, these scripts and stylesheets to be
external. Nightmare??
 Preload scanner: Browsers are smart to optimize when the parsing is
halted by the above operations. It runs a preload scanner for the
further HTML only to identify if there are any any other resources to be
fetched. The preload scanner initiates them meanwhile. Note: The
preload scanner does not modify the DOM at all, it simply cannot.
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Minimize Critical rendering path
 Number of critical resources: Number of resources that may block the
critical rendering path
 Size of the critical resources: Total bytes to get the first render of the
page
 Minimum critical path length: number of round trips. The number of
round trips will depend on the length of the file.
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Important events during the critical
rendering path
 domLoading: the browser is about to start parsing the first received
byte of HTML
 domInteractive: All HTML parsing is complete and DOM Object Model
is created.
 domContentLoaded: This is fired when both DOM and CSS trees are
created. All parser blocking Javascript have finished execution. Render
tree can now be created.
 domComplete: The event is fired when the page and its sub-resources
are downloaded and ready.
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Let’s understand this better…
<html>
<head>
<meta name=“viewport" content="width=device-width,initial-scale=1”>
<link href=”my_style.css" rel="stylesheet">
<title>Example 1</title>
</head>
<body>
<p> Example 1 </p>
</body>
<script src=”my_app.js"></script>
</html>
Number of critical resources: 3
Size of critical resources: Total bytes of HTML +
CSS + Javascript
Minimum critical path length: 2 or more
depending on size
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How to speed up page performance
 Render blocking resources
 CSS
 Javascript & Optimization
 Optimized Images
 Very Low TTFB
 Prioritize content delivery
 Configure the viewport
 Enable Compression
 Leverage browser Caching
 Minified resources
 Use Keep-Alive
 Web fonts
 Use CDN
 Reduce the number of
repaint and reflows
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Render blocking CSS
 Media queries and Media types allow us to mark some CSS
resources as non render blocking.
 The browser does not wait for non render blocking CSS and is
used to optimize the time to first render for a page.
 All CSS resources with render blocking or non blocking will be
downloaded by the browser.
Eg:
<link href="style.css" rel="stylesheet">
<link href="style.css" rel="stylesheet" media="all">
<link href="portrait.css" rel="stylesheet" media="orientation:portrait">
<link href="print.css" rel="stylesheet" media="print">
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Render blocking CSS
 If external CSS resources are small, it is a good idea to embed it within the HTML
page under the style tag.
 Reduce the complexity of the style calculations. Measure the style recalculation
cost and optimize by ensuring only required elements are styled. How to measure
will be covered in a different presentation.
 Avoid CSS @import
 Combine CSS in one file during production deployment
 Don't inline CSS attributes with the HTML
 Programmatically loadCSS which is not applicable to first prioritized HTML
content.
 To identify important CSS use tools like Critical tool. Above the fold CSS.
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Parser blocking Javascript
 When a Javascript code is reached, the browser blocks the execution of
the DOM and hands over the control to Javascript engine till the script is
executed completely and resumes the reconstruction of DOM at the
same point.
 The browser delays the execution of Javascript till the CSS object model
is created
 The location of the script tag within the document is very important
<script src=“myscript.js” async></script>
Tells the browser it does not need to wait for the execution and can start
once the file is fetched from the server. This unblocks the DOM creation.
Placement order of the style and script tag play a very important role for
managing this experience.
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Optimizing Javascript
 Inline JS when necessary reducing network round trip.
 Combine the Javascript file to reduce network calls.
 requestAnimationFrame instead of setTimeout
 Web workers for intensive operations (Do not have access to DOM)
 JS profiler to identify slow Javascript inside Chrome dev tools
 Defer loading javascript. Use of defer keyword may not work across
browsers. Programmatic loading of JS after page load can be used to
download and add javascript not affecting critical above the fold content.
 Use async version of popular 3rd party scripts. Eg: facebook, Google
analytics, twitter etc..
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Optimizing Javascript
 Avoid forced synchronous layouts
 Avoid layout thrashing
 Read and write cycles
 Avoid long running input handlers and style changes in them
(onMouseDown, onChange etc..)
function logBoxHeight() {
box.classList.add(’myCSS');
// Gets the height of the box in pixels
// and logs it out.
console.log(box.offsetHeight);
}
function resizeAllParagraphsToMatchBlockWidth() {
// Puts the browser into a read-write-read-write cycle.
for (var i = 0; i < paragraphs.length; i++) {
paragraphs[i].style.width = box.offsetWidth + 'px';
}
}
FastDOM, a library to reorder the reading and writing and making it async
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 Prefer vector formats: vector images are resolution and scale
independent, which makes them a perfect fit for the multi-device and high-
resolution world.
 Minify and compress SVG assets: XML markup produced by most
drawing applications often contains unnecessary metadata which can be
removed; ensure that your servers are configured to apply GZIP
compression for SVG assets.
 Remove unnecessary image metadata: many raster images contain
unnecessary metadata about the asset: geo information, camera
information, and so on. Use appropriate tools to strip this data.
 WebP is a very promising format, though not supported across all
platforms; they should be used in native applications and its webviews.
Optimized Images
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Optimized Images
 Use base64 for very small images < 5kb to avoid additional calls.
Google is not able to index base64 images which can be addressed
by the meta tag.
<meta property="og:image" content=images/myimage.png>
 Images should be optimized with Lossy filter (eliminates pixel data) ->
Lossless filter (Compression) pipeline.
 Choosing appropriate image format:
 Animated images-> Use GIF
 High quality lossless images-> PNG
 Medium-low quality -> JPEG (experiment with quality based on use case)
 Deliver large image assets size as close to the display size as
possible.
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Very Low Time to first byte (TTFB)
Reasons for high TTFB:
1) Dynamic web page creation: This can
be addressed by deploying web page
caching for dynamic pages.
2) Web server configuration:
.htaccess offers convenience but can be
huge performance issues specially on
apache.
It can be anything from database
performance, slow application logic, slow
routing, slow framework/libraries, resource
starvation (CPU, Memory, Disk).
Less than 100 milliseconds is
superb
200 milliseconds is ideal
Anything else is bad...
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Prioritize content delivery
 Above the fold is a portion of the web page that is visible in
web browser. Always load the critical portion first and defer
loading the rest of the page.
 The CSS delivery can be split into 2 parts to ensure faster page
render.
 Prioritize loading of most important information first and
deferring the rest using lazy loading.
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Configure the Viewport
 <meta name=viewport content="width=device-width, initial-
scale=1">
 Avoid minimum-scale, maximum-scale, user-scalable. These
options negatively impact accessibility and should generally be
avoided.
 If necessary use different styling for small and large screens
using appropriate media queries.
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Enable Compression
 Enabling Gzip compressions as all browsers today understand
Gzip compression.
 Compression is enabled via webserver configuration
 Compression of all HTTP requests results in compression of as
high as 50- 75% in terms of bandwidth and page load time.
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Leverage browser caching
 Network calls are the most expensive operations.
 It is highly recommended an appropriate caching strategy is
adopted for every external resource. Eg: whether the resource can
be cached and by whom, for how long, and if applicable, how it
can be efficiently revalidated when the caching policy expires.
 Cache-Control defines how, and for how long the individual
response can be cached by the browser and other intermediate
caches
 ETag provides a revalidation token that is automatically sent by the
browser to check if the resource has changed since the last time it
was requested.
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Minified resources
 All the HTML, CSS and JS can be minified typically using a
build process to deploy minified resources on the production
environment.
 To minify HTML, try HTMLMinifier
 To minify CSS, try CSSNano and csso.
 To minify JavaScript, try UglifyJS.
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Use keep-alive
 This method allows the browser to use the same connection for
different HTTP requests. This is useful since a web page
involves multiple files like CSS, Images, JS and HTML
 This avoids creating a new connection for every file request.
 Keep-Alive is to be configured on the web server.
 Specially useful in shared environments where keep-alive is
generally turned off.
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Web fonts
 4 types of fonts available for web: WOFF 2, WOFF, TTF, EOT.
 The support for each of them is limited with WOFF being the mostly widely supported
and WOFF 2 is still work in progress.
 EOT and TTF are not compressed by default. Ensure server side compression while
delivery of these fonts.
 WOFF has built in compression ensure optimal compression settings. WOFF2 gives 30%
reduction in file size.
 Separate files for different styles like itlatic, normal. The browser uses the one it needs
Use format hint so that the browser checks and downloads the format it supports.
@font-face {
font-family: 'Awesome Font';
font-style: normal;
font-weight: 400;
src: local('Awesome Font'),
url('/fonts/awesome.woff2') format('woff2'),
url('/fonts/awesome.woff') format('woff'),
url('/fonts/awesome.ttf') format('ttf'),
url('/fonts/awesome.eot') format('eot');
}
url() directive allows us to load external fonts, and
are allowed to contain an optional format() hint
indicating the format of the font referenced by the
provided URL.
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Web fonts
 Unicode range setting:
unicode-range: U+000-5FF; under @font-face
 Manual subsetting
Use the open-source pyftsubset to subset and optimize fonts.
Some font services allow manual subsetting via custom query parameters to manually get a
font subset.
 The browser downloads the required font after creating the render tree. This
causes the web page rendering to be delayed till the font is not available. Devs
can use the font loading API to micro manage this font loading process to get the
required fonts before the render tree. (Browser dependent)
 The devs can inline the font in the CSS to force the browser to download the font
while creating the CSSOM. We should keep a separate CSS file for fonts with
large max age so that the fonts are not downloaded every time a newer version of
CSS is built and deployed. (Be very careful)
 Use HTTP Caching for the fonts.
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Use CDN
 Content delivery network is a network of servers placed in different
geographic locations with your website content and delivered to the clients
from the closest location for reduced network latency.
 CDN can be used for delivery of all static resources: CSS, JS, Images,
HTML
 Popular CNDs
 Cloudflare (has free option)
 Fastly
 Amazon CloudFront
 MaxCDN
 Akamai
 Cachefly
 Keycdn
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Repaint and Reflows
 When part or whole of the render tree needs to be revalidated and the
node dimensions recalculated is called “Reflow”.
 When part of the screen needs to be updated due to changes in style
it requires the browser to paint that area of the screen. This is called
Repaint.
 Code that causes reflow or repaint
 Adding, removing, updating DOM nodes
 Hiding a DOM node with display: none (reflow and repaint) or visibility:
hidden (repaint only)
 Moving, animating a DOM node on the page (repaint)
 Adding a stylesheet, tweaking style properties (reflow or repaint or both)
 User action such as resizing the window, changing the font size, or browser
scrolling
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Expensive operations
 Browsers optimizes the reflows and repaint operations by performing
in batches. Though are certain functions that causes the browser to
perform the reflow and/or repaints immediately and hence should be
used very carefully.
Element: clientHeight, clientLeft, clientTop, clientWidth, focus(), getBoundingClientRect(), getClientRects(),
innerText, offsetHeight, offsetLeft, offsetParent, offsetTop, offsetWidth, outerText, scrollByLines(),
scrollByPages(), scrollHeight, scrollIntoView(), scrollIntoViewIfNeeded(), scrollLeft, scrollTop, scrollWidth
Frame, Image: height, width
Range: getBoundingClientRect(), getClientRects()
SVGLocatable: computeCTM(), getBBox()
SVGTextContent: getCharNumAtPosition(), getComputedTextLength(), getEndPositionOfChar(),
getExtentOfChar(), getNumberOfChars(), getRotationOfChar(), getStartPositionOfChar(),
getSubStringLength(), selectSubString()
SVGUse: instanceRoot
Window: getComputedStyle(), scrollBy(), scrollTo(), scrollX, scrollY, webkitConvertPointFromNodeToPage(),
webkitConvertPointFromPageToNode()
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Recommendations for minimizing
Repaint and Reflow
 Don’t read and write to style sheet very quickly. Batch the query
state and change state statements
 Don’t change individual style one by one if there are multiple
changes, use cssText property as it batches the application of
change
 Don’t ask for computed style multiple times. Store in local
variable and reuse them specially for expensive operations.
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PageSpeed module – Apache,
Nginx
 Page speed module is developed by Google and can be
configured with Apache or Nginx to perform web page
optimization best practices.
 These optimizations are performed by configuring filters in the
page speed module.
 Categories of filters:
 Optimize Caching
 Minimize round trip times
 Minimize request overhead
 Minimize payload size
 Optimize Browser rendering
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What’s next?
 How to measure page performance using tools?
 JavaScript Internals
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References
 https://developers.google.com/web/fundamentals/performance/?hl=en
 https://developers.google.com/speed/docs/insights/about
 https://jonsuh.com/blog/need-for-speed-2/
 https://classroom.udacity.com/courses/ud884/
 http://www.phpied.com/rendering-repaint-reflowrelayout-restyle/
 http://gent.ilcore.com/2011/05/how-web-page-loads.html
 http://gent.ilcore.com/2011/03/how-not-to-trigger-layout-in-webkit.html
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