The document discusses optimizing the critical rendering path (CRP) of a web page. The CRP refers to the steps between receiving HTML, CSS, and JavaScript and rendering pixels on the screen. These steps include parsing HTML to build the DOM tree, parsing CSS to build the CSSOM tree, combining them into a render tree, running layout to compute geometry, and painting to the screen. Optimizing the CRP means minimizing the time spent in these steps. Some tips include getting CSS to the client fast, eliminating blocking JavaScript from the CRP, and focusing on above-the-fold content. Tools like critical CSS extraction can help optimize the CRP.

2. Optimizing the CRP
(Critical Rendering Path)

3. Vlad Zelinschi
Frontend Engineer at Yonder
@vladzelinschi

4. “One does not simply optimizes the
critical rendering path so easily…”

5. Reasons for being here
• Easy to construct large apps and just … “go with the
flow”
• Performance should be in your head right from the
start (this does not mean over-optimizing early on!!)
• Demanding clients, demanding users, demanding
developers, high standards

6. SPEED IS A
FEATURE

7. What is the CRP?

8. “[…] what happens in these intermediate steps
between receiving the HTML, CSS, and
JavaScript bytes and the required processing to
turn them into rendered pixels - that’s the critical
Google Developers
rendering path.”

10. Candy break …

11. Enter The Network …
• DNS Lookup to resolve the hostname to IP address
• New TCP connection (if necessary)
• TLS/SSL handshake happening (worst case
scenario)
• HTTP request for index.html

12. Some TCP knowledge
• TCP is designed to probe the network to figure out
the available capacity
• TCP does not use the full capacity from the start
• TCP Slow Start - for congestion control and
avoidance
• For the first roundtrip ~ 14KB (10 TCP packets)

14. let’s get the index.html

17. What browsers do …
• Conversion: from raw bytes to individual characters
(based on encoding - ex: UTF-8)
• Tokenizing: converts strings of characters into distinct
tokens specified by the W3C HTML5 standard
• Lexing: tokens are converted into “objects” which
define their properties and rules
• DOM construction: relation between tags, tree-like
construction

19. same for CSS …

24. then the render tree …

26. Render, Layout, Paint
• Final stages before getting something on screen
• The Layout calculates the exact position and size of
the elements in the viewport
• The render tree construction, position and size
calculation are captured with the “Layout” event in
the Timeline (Chrome Dev Tools)
• Paint - converts the render tree to actual pixels

27. Recap
• Process HTML markup and build the DOM tree
• Process CSS markup and build the CSSOM tree
• Combine the DOM and CSSOM into a render tree
• Run layout on the render tree to compute geometry
of each node
• Paint the individual nodes to the screen

28. Optimizing the critical rendering path is the
process of minimizing the total amount of time
spent in steps one through five in the previous
slide.

29. Perf rules to keep in mind
• HTML is parsed incrementally
• CSS is render blocking (can’t construct the CSSOM
and the render tree without it)
• CSS is not incremental (we must wait for the entire
file to download)

31. What about JavaScript?

33. JavaScript - friend and foe
• The location of the script in the document is significant
• DOM construction is paused when a script tag is
encountered and until the script has finished executing
(JS is parser-blocking)
• JavaScript can query and modify the DOM and
CSSOM
• JavaScript execution is delayed until the CSSOM is
ready

34. How can we have pixels
on screen after just one
RTT (round trip time)?

39. Optimizing the CRP
• Get CSS down to the client as fast as possible
• Eliminate blocking JavaScript from the CRP (your
friend, async)
• Aim for the fastest domContentLoaded time (DOM
and CSSOM are ready)

40. Optimizing the CRP
• Concentrate on the above the fold content - ATF
(what the user sees first)
• Inline CSS and JS for ATF (remember the 14KB
limit?)
• Put your critical assets on the same domain
• Consider minifying HTML

41. Tools for the trade
• https://github.com/filamentgroup/grunt-criticalcss
• https://github.com/bezoerb/grunt-critical
• https://github.com/filamentgroup/loadCSS
• https://github.com/filamentgroup/loadJS
• https://developers.google.com/speed/pagespeed/

42. Real world example
https://www.fasetto.com/

43. Thank you!

44. Q&A anyone?

46. Please fill the online evaluation form after event
Optimizing the CRP (Critical Rendering Path)
Vlad Zelinschi
Frontend Engineer at Yonder