Understanding the critical rendering path, rendering pages in 1 second

A popular metric commonly used in performance measurement and evaluation, is the total page load time. This metric, obviously, has it’s importance when evaluating performance, as it states the full download time, to get your website and all of it’s assets.

But from a user’s point of view, more important benchmarks arise, despite the fact that the full download time of a page is still important, the user is much more aware of the time it takes for the page to become usable, or the time it takes to render the main content, so that he can access those first glances of information.

Some usability metrics, that were initially defined by Norman Nielsen, state that, after only one second with no response, there’s a mental context switch in our brains, that’s the moment when we stop focusing in the task at hand and start thinking about other subjects.

0–100 ms — Instant feel, constant flow;
100–300 ms —
Slight percetible delay;
300–1000 ms — Loss of task focus, perceptible delay;
1 s+ — Mental context switch;
10s+ — User leaves;

In a modern website or application, loading a page in one second seems like a daunting and impossible task, we have to handle, amongst other things: Javascript, CSS, and third party scripts.

But we don’t need to fully load a page in one second, what we need to do, is to get to the most important content, to get some usable piece of information to the user in one second, in order to maintain flow and give a feel of instant execution, an instant experience.

In order to achieve this, we first need to fully understand what’s needed for a browser to render content to the screen.

How does the browser rendering engine work?

In order to render content the browser has to go through a series of steps:
1. Document Object Model(DOM)
2. CSS object model(CSSOM)
3. Render Tree
4. Layout
5. Paint.

1. Document Object Model

To process a html file and get to the document object model event(DOM) the browser has to go through 4 steps:

1. Convert bytes to characters
Identify tokens
Convert tokens to nodes
Build DOM Tree

This entire process can take some time, specially if there is a large amount of HTML to process. This means that the initial file size of your DOM will have a performance cost.

In order to measure the full time needed for this process, you can record a timeline on chrome devtools while your page is loaded.

Timeline recorded with Chrome DevTools

In the timeline above you can see that the browser initially sends a request for the html, then it starts receiving the response in chunks of data, and initializes the html parser, as the parser finds any links for CSS or Javascript, it immediatelly sends a request for them, after that it also sends requests for all the other assets found in the rest of the page.

When this process is finished the browser will have the full content of the page, but to be able to render the browser has to wait for the CSS Object Model, also known as CSSOM event, which will tell the browser how the elements should look like when rendered.

2. CSS Object Model

Just as with HTML, the CSS rules need to be converted into something that the browser understands, so these rules go through the same steps as the document object model.

1. Convert bytes to characters
Identify tokens
Convert tokens to nodes

In this stage the CSS parser goes through each node and gets the styles attributed to it.

CSS is one of the most important elements of the critical rendering path, because the browser blocks page rendering until it receives and processes all the css files in your page, CSS is render blocking

3. The Render Tree

This stage is where the browser combines the DOM and CSSOM, this process outputs a final render tree, which contains both the content and the style information of all the visible content on the screen.

4. Layout

This stage is where the browser calculates the size and position of each visible element on the page, every time an update to the render tree is made, or the size of the viewport changes, the browser has to run layout again.

5. Paint

When we get to the paint stage, the browser has to pick up the layout result, and paint the pixels to the screen, beware in this stage that not all styles have the same paint times, also combinations of styles can have a greater paint time than the sum of their parts. For an instance mixing a border-radius with a box-shadow, can triple the paint time of an element instead of using just one of the latter.


Dealing with Javascript

Javascript is a powerful tool that can manipulate both the DOM and CSSOM, so to execute Javascript, the browser has to wait for the DOM, then it has to download and parse all the CSS files, get to the CSSOM event and only then finally execute Javascript.

When the parser finds a script tag it blocks DOM construction, then waits for the browser to get the file and for the javascript engine to parse the script, this is why Javascript is parser blocking.

Here we can understand the importance of the CSSOM event in the critical rendering path, this single event is blocking rendering and Javascript execution.

There are only two cases when Javascript does not block on CSSOM:
1. Inlined scripts above the css files ‘<link>’ in the ‘<head>’;
2. Async scripts.

Make all Javascript async

Async scripts don’t block DOM construction and don’t have the need to wait for the CSSOM event, this way your critical rendering path stays free from Javascript interference. This is a crucial part of optimising for the critical rendering path.

Optimizing the critical rendering path

By now we can conclude that in order to render a page as fast as possible, we need to beware of: the initial html file size, optimize CSS delivery and take Javascript out of the rendering path by making all scripts async.

Understanding the critical rendering path

Let’s say that we have a page that has one CSS and one javascript file and that we’ve made the javascript async taking it out of the rendering path equation.

In this example we have two critical resources, the html and the css file, which make for a total size of 46kb in our critical path, we also need 2 requests in order to get everything we need to render the page (the 2 requests are the HTML and CSS, the Javascript file is marked as async so it’s out of the rendering path equation).

How would we optimize this?

First of all, we need to make our critical assets as small as possible by minifying and compressing both the html and css. We can further optimize this resources by making use of html and css obfuscation. Obfuscation is the process of going through all of your class names in both the html and css, and converting something like ‘.header__nav — fixed’ into ‘.a’. This process can achieve big gains, specially when using long naming conventions such as BEM. This technique is already in use for sometime now in the Javascript world, but it’s use on HTML and CSS is still very recent.

Some tools available:

Next step optimising css delivery

With our resources optimized we need to get them to the client as fast as possible. A good strategy is to inline our css, so that we can get it to the client with the first html request, allowing the browser to get to the CSSOM event with only one request.

But in this case our first request would get a little to big with 46kb. The page could still render faster but in some cases a big initial request can make things worst.

This can be tricky specially when dealing with responsive websites (should we inline responsive grids?, should we inline for mobile and desktop? Always test it first!)

A good strategy is to inline only the css for the header and the main module of the page, and downloading the remaining css async. You can hide the unstyled content of the page, while the client waits for the reamining CSS, this is made to avoid the flash of unstyled content.

Tools available:

If you’re going down this path and you’re using web fonts, it’s an absolute must to have your webfonts async. You don’t want to render the styles immediately and have your users waiting on the fonts to see the text, which is probably the most important part of your content.

More on async webfonts:

Achieving the 1s render

So let’s say that we’ve done all the optimizations mentioned above, will our page render in one second?

…the server can send up to 10 TCP packets on a new connection (~14KB) in first roundtrip, and then it must wait for the client to acknowledge this data before it can grow its congestion window and proceed to deliver more data.
Due to this TCP behavior, it is important to optimize your content to minimize the number of roundtrips required to deliver the necessary data to perform the first render of the page. Ideally, the ATF (above the fold) content should fit under 14KB — this allows the browser to paint the page after just one roundtrip…

In order to make our page render in 1s we need to make our critical html and css fit in approximately 14kb. If we follow the logic mentioned above, this would be the header and the main module of the page.


After achieving the 14kb goal, we need to render the content, but the process of parsing the HTML, CSS, and executing Javascript takes time and resources, in a 3g connection we have ~600ms of 3g networking overhead (4g networks can reduce this overhead), then we need to wait for the server response, which can be ~200ms and then we are left with 200ms to parse the html and css, so it really helps to keep things simple(this is one of the advantages of design principles such as flat design and mobile first).

If you’ve managed to get through the whole process and made all the optimisations mentioned above, congratulations you should now have devivered an instant experience.


Understanding how a browser works and how it gets the content needed to render the page, is essential for optimising the critical rendering path.
Also simplicity in design and thinking about performance since day one, are essential elements to achieve the holy grail of the instant experience. We cannot deliver the main content of the page in one second if we expend that budet with just the header or with extremely complex styling.
Still it’s always better to render something as fast as possible to the page, than having your users staring at a blank screen.
We’ll talk again when HTTP2 usage goes mainstream and everything changes again!