CSS and the Critical Path - CSSConfEU September 2014

For a website to feel instant, a perceptible response to user input must be provided within hundreds of milliseconds.

This perceived speed of your website relies heavily on the browser being able to paint to your user’s screen. For this they must construct a “render tree” which consists of the DOM and the often forgotten CSSOM. The critical path which the browser takes to gather this information is the only thing standing between your server and the user’s screen.

Using new research and real world examples, Patrick Hamann will cover a range of techniques – from the controversial to bleeding edge – the Guardian are using to make the CSS of their next generation website load as fast as possible, and ultimately taking it off the critical path. Breaking the news to the user within 1000ms.

During the talk you will discover why performance matters, what are the common performance bottlenecks in the browser from networking to painting and learn how to best optimise and monitor each stage of the critical path to create fast loading websites.

Transcript

1. CSS and the critical path @patrickhamann - CSSConf EU - September 2014
2. next.theguardian.com
3. 105million !
4. " # $ 6000
6. 0 7500 15000 22500 30000 Jan Feb Mar Apr May June July August 10% Percentile Median 90% Percentile Source: HTTPArchive - BigQuery - September 2014 Average SpeedIndex from top 10,000 websites
7. Maslow's hierarchy of user needs ! % & ' We surveyed 3000 users About 17 key product drivers They rated speed 2nd most important Only after easy to find content
8. Time & user perception Delay User perception 0–100 ms Instant 100–300 ms Small perceptible delay 300–1000 ms Machine is working 1,000+ ms Likely mental context switch 10,000+ ms Task is abandoned Source: Ilya Grigorik – High-Performance Browser Networking
9. 1000ms
10. A performance budget https://flic.kr/p/iQ69Kg
11. Browser rendering 101
12. HTTP (TTFB)
 DNS Lookup Content download Initial connection (Socket)
13. Source: Google - Pagespeed Insights
14. Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM
15. Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM DOM
16. Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM DOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM
17. Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM DOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint DOM CSS CSSOM
18. Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM DOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM Network JavaScript Render tree Layout Paint HTML DOM CSS CSSOM
19. Request page Network Render GET html Build DOM response Build CSSOM Render page GET css GET js response response idle idle Async Render blocking Run JS
20. Request page Network Render GET html Build DOM response Build CSSOM Render page GET css GET js response response idle idle Async Render blocking Run JS
21. JS CSS HTML CSS & JS § HTML async DOMContentReady Start render
22. Get the CSS down as soon as possible.
23. CSS and the critical path
24. What is your critical CSS?
25. Popular content ✘ Sharing ✘ Comments ✘ Related content ✘ Commercial components ✘ Article ✔
26. Final progressively enhanced page Initial page payload
27. What if we inlined our critical CSS?
28. Inline
29. WTF%$?!
31. 0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3
33. localStorage
34. Is modern browser? Requet with <link> Request with <link> CSS in localStorage? localStorage space? Request with AJAX cache in localStorage inject into <head>
38. Load time First byte Start render Visually complete 3.366s 0.204s 1.113s 3.700s Before Load time First byte Start render Visually complete 3.017s 0.262s 0.759s 2.00s After
39. Use MD5 hash for cache key WTF no stylesheets?
41. Cookie
42. Initial request HTML response with inline CSS Render page Request global CSS with AJAX Set cookie Initial request HTML response with default <link> Render page Does request contain cookie?
43. Monitor
44. WARNING!
45. assets radiator
46. speedcurve
47. Alternatives
49. What does the future hold?
50. Resource priorities Yay!
51. http/2
52.  ) HTML " ) CSS  " HTTP/1.1 * HTML & CSS  " HTTP/2 Server push
53. service worker ServiceWorker
54. ServiceWorker  " )  " ) )
55. Takeaways
56. Inline critical CSS and defer all other non-critical assets to avoid render blocking.
57. Cache aggressively.
58. Set a performance budget.
 Measure, optimise & repeat!
59. Thank you. @patrickhamann - CSSConf EU - September 2014 github.com/guardian/frontend + bit.ly/cssconf-critical-path ,