More Than You Ever Wanted to Know About Resource Hints

More Than You Ever Wanted to Know About Resource Hints
Harry Roberts • @csswizardry

Hi, I’m Harry Consultant Performance Engineer Leeds, UK csswizardry.com @csswizardry

i Pro-Tip / Short Summary

! Caveat / Warning / Bug

Resource Hints

“ “These primitives enable the developer […] to assist the
user agent in the decision process of which origins it should connect to, and which resources it should fetch and preprocess to improve page performance.”

i Single lines of HTML that can dramatically speed up
your site

csswz.it/rh

Overview 1.dns-prefetch 2.preconnect 3.prefetch 4.preload 5.subresource 6.prerender

dns-prefetch

csswz.it/dns-prefetch

i Resolve the IP address for a given domain ahead
of time

i When you know the domain but not the URL

154 <iframe 155 src="https://www.youtube.com/embed/5g8a9luSZVI" 156 width="560" height="315"> 157 </iframe>

5g8a9luSZVI

DNS Lookup

216.58.198.110

“ “[…] common names […] can answer in closer to
80–120ms. […] an uncommon name […] can average closer to 200–300ms.” — csswz.it/2GuZo21

“ “More interestingly, for any of these queries that access
the internet, dropped packets, and overworked (under provisioned) name resolvers, regularly increases the total resolution time to between 1 and 10 seconds.” — csswz.it/2GuZo21

@andydavies

! dns-prefetch is implemented as prefetch in IE9…

preconnect

csswz.it/preconnect

i Resolve the IP address and open a TCP/TLS connection
for a given domain ahead of time

i When you know the domain but not the URL

5g8a9luSZVI

TCP Handshake TLS Negotiation

hpbn.co

preconnect with  dns-prefetch fallback

! Breaks in Safari

@andydavies

! Be judicious with preconnect

Be Judicious Only warm up frequent, signiﬁcant, and likely origins
Don’t warm up fourth, ﬁfth, sixth party origins Opening many connections can have a CPU and battery cost Chrome can only conduct six simultaneous DNS resolutions

! Firefucked

prefetch

csswz.it/prefetch

i A file needed for subsequent navigation

<link rel="prefetch" href="/assets/video-player.js" />

i Download the file and drop it into HTTP cache
for later usage

“ “The user agent SHOULD NOT apply preprocessing on the
response and MUST NOT automatically execute or apply it against the current page context.”

<link rel="prefetch" href="/assets/video-player.js" /> 
<script src="/assets/video-player.js"></script> Downloaded by this… … executed by this.

Caching prefetch will not execute or otherwise process the resource
It will drop it into HTTP cache as per its caching headers Except…

“ “…those with the no-store Cache- Control header. A resource
will be revalidated before use if there is a Vary response header, no-cache Cache- Control header, or if the resource is more than ﬁve minutes old.” — csswz.it/nostate-prefetch

i Fetched with lowest possible priority

i In-flight prefetches persist across navigations

! …prefetch acts like dns-prefetch in IE9

preload

csswz.it/preload

i A mandatory fetch for a file needed for the
current navigation

i A way to surface late- discovered resources

Late-discovered resource?

HTML CSS Font } 1000ms

HTML CSS Background } 1250ms

HTML CSS Font JS VDOM } 1350ms

preload helps the browser find them sooner

HTML CSS Font JS VDOM } 750ms

The as Attribute

"audio" "document" "embed" "fetch" "font" as="image" "object" "script" "style" "track"
"video" "worker"

“ “The attribute is necessary to guarantee correct prioritization, request
matching, application of the correct CSP3 policy, and setting of the appropriate Accept request header.”

! Don’t try to be sneaky

i Priority Hints will help

The type Attribute

The crossorigin Attribute

“ “Preload links for CORS enabled resources, such as fonts
or images with a crossorigin attribute, must also include a crossorigin attribute, in order for the resource to be properly used.”

i Keep an eye on Console

! Beware Google Chrome

Chrome Issues Chrome over-prioritises preload Dispatches preloads before other critical
resources Often returns less-critical resources sooner

@andydavies

0.4s slower to start render @andydavies

subresource

i Precursor to preload…

! …but completely deprecated.

SUBRESOURCE

“ “…not useful, proprietary, and buggy.” — Yoav Weiss, csswz.it/2VOEyQc

prerender

csswz.it/prerender

i Download and build entire webpages in the background…

i …kinda…

i …it’s complicated.

PRERENDER

Problematic Prerender Huge memory footprint with rendering whole new pages
Bandwidth usage spikes for site and user Register multiple analytics hits, ad impressions How do we handle timers, HTTP auth, interstitials, autoplay media? How do we handle animations? Do we expect the carousel to start running? Or do we write more code to wait for page visibility? What if we get MitM and are made to prerender a malicious page? How would the user know? They wouldn’t!

Prerender is dead,  long live prerender!

Prerender is dead,  long live prerender! NoState Prefetch

i prerender is the API; NoState Prefetch is the mechanism

! prerender will not render anything

! prerender will not render anything !

NoState Prefetch

i A recursive prefetch

A B B1 B2 B3 B4 B5

<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8" /> <meta name="viewport"
content="width=device-width, minimum-scale=1.0" /> <title>Prerender</title> <link rel="prerender" href="https://csswizardry.com" /> </head> <body> <h1>Prerender</h1> </body> </html>

i Fetched with lowest possible priority

Tips, Tricks, and Gotchas

i Deploy as HTTP Headers

@andydavies

! Edge only supports HTTP header for preconnect

! Edge doesn’t support HTTP header for preload

i Generate Dynamically

instant.page

! Have an escape hatch

if ($omgWeAreBeingDDoSed == false) { <script src="https://instant.page/1.1.0" defer></script> }

! You Need a Scheme

All Together!

Implementing Resource Hints 1.Identify a key page 2.Audit important assets
and origins 3.Assess likely user ﬂows 4.Design bespoke Resource Hint strategies

Web font Likely next navigation Critical third party

Web font Likely next navigation Critical third party JS image
gallery

Resource Hint Strategy 1.HTTP header preconnect to cloudinary 2.Regular preconnect
for other third parties 3.preload to discover web font sooner 4.prefetch for gallery.js 5.prerender or prefetch for likely next navigation

// HTTP response header link: <https://res.cloudinary.com>; rel=preconnect

<link rel="preconnect" href="https://www.google-analytics.com">
<link rel="dns-prefetch" href="https://www.google-analytics.com">

<link rel="stylesheet" href="/assets/app.css" /> <link
rel="preload" href="/assets/webfont.woff2" as="font" type="font/woff2" crossorigin />

<link rel="prefetch"
href="/assets/gallery.js" />

<link rel="prerender" href="/products/one.html"
/> <link rel="prerender" href="/products/two.html" /> <link rel="prerender" href="/products/three.html" />

Thank You! @csswizardry [email protected] speakerdeck.com/csswizardry harry.is/for-hire

More Than You Ever Wanted to Know About Resource Hints

More Than You Ever Wanted to Know About Resource Hints

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