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

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

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i Pro-Tip / Short Summary

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! Caveat / Warning / Bug

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Resource Hints

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“ “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.”

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

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... ...

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csswz.it/rh

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

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dns-prefetch

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csswz.it/dns-prefetch

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

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i When you know the domain but not the URL

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154 157

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5g8a9luSZVI

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DNS Lookup

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216.58.198.110

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

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“ “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

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@andydavies

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! dns-prefetch is implemented as prefetch in IE9…

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preconnect

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csswz.it/preconnect

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

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i When you know the domain but not the URL

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5g8a9luSZVI

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TCP Handshake TLS Negotiation

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hpbn.co

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preconnect with
 dns-prefetch fallback

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! Breaks in Safari

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@andydavies

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! Be judicious with preconnect

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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

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! Firefucked

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prefetch

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csswz.it/prefetch

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i A file needed for subsequent navigation

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

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

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Downloaded by this… … executed by this.

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

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“ “…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

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i Fetched with lowest possible priority

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i In-flight prefetches persist across navigations

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! …prefetch acts like dns-prefetch in IE9

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preload

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csswz.it/preload

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

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i A way to surface late- discovered resources

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Late-discovered resource?

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HTML CSS Font } 1000ms

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HTML CSS Background } 1250ms

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HTML CSS Font JS VDOM } 1350ms

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preload helps the browser find them sooner

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HTML CSS Font JS VDOM } 750ms

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The as Attribute

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

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

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! Don’t try to be sneaky

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i Priority Hints will help

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The type Attribute

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The crossorigin Attribute

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“ “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.”

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i Keep an eye on Console

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! Beware Google Chrome

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

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@andydavies

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@andydavies

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0.4s slower to start render @andydavies

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subresource

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i Precursor to preload…

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! …but completely deprecated.

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SUBRESOURCE

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

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prerender

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csswz.it/prerender

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i Download and build entire webpages in the background…

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i …kinda…

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i …it’s complicated.

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PRERENDER

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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!

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Prerender is dead,
 long live prerender!

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Prerender is dead,
 long live prerender! NoState Prefetch

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i prerender is the API; NoState Prefetch is the mechanism

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! prerender will not render anything

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! prerender will not render anything !

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NoState Prefetch

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i A recursive prefetch

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A B B1 B2 B3 B4 B5

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Prerender

Prerender

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i Fetched with lowest possible priority

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Tips, Tricks, and Gotchas

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i Deploy as HTTP Headers

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@andydavies

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! Edge only supports HTTP header for preconnect

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! Edge doesn’t support HTTP header for preload

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i Generate Dynamically

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instant.page

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! Have an escape hatch

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if ($omgWeAreBeingDDoSed == false) { }

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! You Need a Scheme

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==

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All Together!

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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

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Web font Likely next navigation Critical third party

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Web font Likely next navigation Critical third party JS image gallery

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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

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// HTTP response header link: ; rel=preconnect

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