---
layout: post
title: "blocking=render: Why would you do that?!"
date: 2024-08-14 12:45:11
categories: Web Development
main: "https://csswizardry.com/wp-content/uploads/2024/08/blocking-status.png"
meta: "Why on earth would you make something render-blocking?!"
faq:
  - question: "Why would be need a blocking=render attribute?"
    answer: "Generally, we don’t! The primary use case thus far is to grant scripts the ability to be render- but not parser-blocking, which is not currently possible. This would be particularly helpful for A/B testing libraries who currently rely on anti-flicker snippets to achieve the results they want."
---

WebKit have recently announced their [intent to
implement](https://github.com/WebKit/WebKit/pull/32022) the `blocking=render`
attribute for `<script>` and `<style>` elements, bringing them in line with
support already available in
[Blink](https://issues.chromium.org/issues/40205664) and [generally positive
sentiment in Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1751383).

The `blocking=render` attribute allows developers to explicitly mark a resource
as render blocking, but… why on earth would you want to do that?!

The short answer is: **generally, you wouldn’t**. Unless you _know_ you need
this behaviour, you don’t need it.

But how do you know if you do need it? Read on…

## What is `blocking=render`?

The spec says:

> A blocking attribute explicitly indicates that certain operations should be
> blocked on the fetching of an external resource. The operations that can be
> blocked are represented by possible blocking tokens, which are strings listed
> by the following table […]  
> — [2.5.8 Blocking attributes](https://html.spec.whatwg.org/multipage/urls-and-fetching.html#blocking-attributes)

Currently, there is only one token specified: `render`. The spec is extensible
so that other values could be added as the need arises—[potential scenarios that
have been
discussed](https://gist.github.com/xiaochengh/fae2b549b3d37454beeb9028a829f4bd#other-operations-to-blockunblock-on)
include `parse`, `load`, and even a negation to encourage the opposite, such as
`blocking=!render`.

## Blocking Status

Generally speaking, when loading resources into web pages, there are three
possible blocking states:

1. **Non-blocking:** From a performance perspective, this is the most desirable.
   The resource is fetched and processed asynchronously while the browser is
   free to work on whatever other tasks there may be. The two key tasks that are
   not blocked are _rendering_ and _parsing_.
2. **Render blocking:** The next-best option for the performance conscious is
   render blocking. Files that are render blocking prohibit the browser from
   presenting the page, but do permit the browser to at least construct it.
3. **Parser blocking:** The worst case scenario is a file that prevents the
   browser from even building the page. All parsing and rendering is blocked
   while the resource is fetched. Files that are parser blocking are inherently
   also render blocking—the browser can’t present a page that it can’t even
   construct.

Visually, this is how that process looks for each scenario:

<figure>
<img src="{{ site.cloudinary }}/wp-content/uploads/2024/08/blocking-status.png" alt="Comparison of non-blocking, render-blocking, and parser-blocking resources in web performance. A visual breakdown of how different loading strategies affect rendering, parsing, and blocking behaviour in the browser." width="750" height="424">
<figcaption>
  A non-, render-, and parser-blocking file in an HTML document. Imagine the
downloading file (pink) is in the <code>&lt;head&gt;</code>—even though you can
never see <code>&lt;head&gt;</code> tags or their children, they still get
rendered just like any other HTML, they’re just set to <code>display:
none;</code>. That said, these diagrams also apply to a downloading file (pink)
that is in the middle of the <code>&lt;body&gt;</code>. HTML is parsed
line-by-line and is very predictable. We ❤️ HTML.
</figcaption>
</figure>

### Blocking Files

The two main file types that impact the blocked status of a web page are
stylesheets and scripts. In their default states:

* `<link rel=stylesheet href=app.css>`: This will block the rendering of
  subsequent content, but not its parsing. The browser is free to continue
  parsing the HTML and building out the DOM, but cannot display any of it until
  `app.css` is fully fetched and parsed. **Stylesheets are render blocking.**
* `<script src=app.js></script>` This will block parsing (and therefore also
  rendering) of subsequent content. The browser may not parse or construct any
  DOM until `app.js` is fully fetched and parsed, at which point it now has two
  tasks ahead of it: build the DOM and render it. **Scripts are parser
  blocking.**

All other file types are, by default, non-blocking.

<small>The pedant in me wants to point out that even inline `<script>` and
`<style>` are still technically parser blocking. Colloquially, we refer to them
as non-blocking, but even for the handful of milliseconds that the browser is
parsing either the JS or CSS contained in them, it’s blocked from doing anything
else.</small>

### `async`, `defer`, and `type=module`

Without going into too much detail, the presence of any of these attributes on
a `<script>` will cause it to fall into the first camp: non-blocking. Therefore,
`<script>`s can occupy either extreme: non-blocking, the fastest option; or
parser blocking, the slowest option.

**The primary use-case for `blocking=render` is to grant `<script>`s access to
the middle option: render- but not parser-blocking.**

Let’s look at two examples of putting `blocking=render` to use.

## Blocking Web Fonts

<ins>I wrote this entire section before [Ryan Townsend](https://twnsnd.com/)
pointed out that `blocking` specifically for `rel=preload` was [removed from the
spec](https://github.com/whatwg/html/issues/7896). I’m keeping the following for
posterity, but this does not currently work in any implementation.</ins>

This is one of the least compelling examples, in my opinion. Also, for this to
work, the `blocking` attribute needs specifying for `<link>` elements<del>,
which is currently only possible in Blink</del>. But let’s take a look
anyway…

Imagine you’ve built a simple countdown or stopwatch app:

<figure>
<img src="{{ site.cloudinary }}/wp-content/uploads/2024/08/foft.gif" alt="A digital stopwatch running an HTML file, displaying ‘00:51’ in large green text before unceremoniously changing its typeface and continuing to count down." width="750" height="432">
<figcaption>The change from fallback font to web font causes a very noticeable
change in UI. This might be unacceptable.</figcaption>
</figure>

Given a UI such as this, even with the best will in the world, the switch from
any fallback font to the intended web font is quite a leap. Is it _too much_? If
you decide it is, you could block on the `preload` of that font (if you were
`preload`ing it in the first place). That would look like this:

```html
<link rel=preload
      as=font
      href=font.woff2
      crossorigin
      blocking=render>
```

Typically, I would strongly recommend not blocking rendering on web fonts. Using
the relevant `font-display` to ensure that text can render as soon as possible
is almost always the correct thing to do: reading something in the ‘wrong’ font
is better than reading nothing at all.

However, in scenarios where a flash of fallback font (FOFT) might be
particularly jarring—or create severe layout shifts—then perhaps waiting on the
web font might (_might_) be the right thing to do. Maybe. I’m not actively
recommending it.

Note that almost the exact same behaviour could be achieved by adding
`font-display: block;` to the relevant `@font-face` rule, but `blocking=render`
<del>provides</del> <ins>would have provided</ins> two distinct additions:

1. `font-display: block;` will time out after three seconds, whereas
   `blocking=render` has no such timeout. In that sense, it’s much more
   aggressive.
2. `font-display: block;` will still render the current UI, only without text—a
   flash of invisible text (FOIT). `blocking=render` won’t render anything at
   all.

If a web font _is_ your content (which, for 99.999% of you, it isn’t), you might
want to maybe use `blocking=render`. But even then, I wouldn’t.

<ins>Interestingly, Chrome exhibits `blocking=render`-style behaviour on
web-font `preload`s already. It’s non-standard behaviour, but Chrome [<q>will
make font preloads block rendering until they finish or until
a timeout</q>](https://chromestatus.com/feature/5087982807154688). This is
already happening and you don’t need `blocking=render`.[^1]</ins>

## A/B Testing and Experimentation

`blocking=render`’s application in client-side A/B testing is, for me, its most
compelling use-case.

Client-side A/B testing tools work by altering the DOM and presenting a variant
of a component to a user. In order for this to work, the original DOM must
already be constructed (you can’t alter a DOM if you don’t have one), so there’s
an aspect of doing the work twice. A problem arises if and when a user actually
_sees_ that work happening twice. It’s a jarring experience to see one version
of a hero change to something completely different in front of your eyes, and it
may even influence the outcome of the experiment itself.

To circumvent this, many A/B testing tools implement what is known as an
anti-flicker snippet. They deliberately hide the page (visually) until the
variants have been constructed, or a timeout is met—whichever happens sooner.

This is the anti-flicker snippet from the [now
defunct](https://support.google.com/analytics/answer/12979939?hl=en) Google
Optimize.

```html
<!-- Anti-Flicker Snippet -->
<style>
  .async-hide { opacity: 0 !important }
</style>

<script>
  (function(a,s,y,n,c,h,i,d,e) {
    s.className+=' '+y;
    h.start=1*new Date;
    h.end=i=function(){
      s.className=s.className.replace(RegExp(' ?'+y),'')
    };
    (a[n]=a[n]||[]).hide=h;
    setTimeout(function(){i();h.end=null},c);
    h.timeout=c;
  });
  (window, document.documentElement, 'async-hide', 'dataLayer', 4000, {'GTM-XXXXXX':true});
</script>
```

This snippet works by applying the class `async-hide` to the `<html>` element
(`document.documentElement`). This aggressively sets `opacity: 0;` so that the
page is rendered, only invisibly. The class is then removed either when the A/B
tool’s work is done, or a `4000`ms timeout is reached—whichever is first.

One immediate failing with this is that an invisible page is still interactive,
and users could still click on or interact with elements inadvertently. The page
_is_ rendered, but invisibly. `blocking=render` ensures that the page is not
rendered at all, and therefore can’t be interacted with.

Another problem is that we’re going through more paint cycles than we need to:
paint the page invisibly, modify it, paint it again visibly… It would be nicer
to hold off painting anything at all until we have all of the relevant
information about what to paint. `blocking=render` gives us this ability.

A further issue is the big-reveal phenomenon: with an anti-flicker snippet, the
page is totally invisible until it’s totally visible. Behind the `opacity: 0;`,
there may well have been a progressive render of the page—which is a familiar
and good user experience—but a user didn’t benefit from it. Anti-flicker
snippets eschew this behaviour and take an all-or-nothing approach: nothing,
nothing, nothing, _everything_.

<figure>
<img src="{{ site.cloudinary }}/wp-content/uploads/2024/08/anti-flicker.png" alt="Progressive rendering sequence of a webpage displaying a web performance consultancy service. The timeline from 0.8s to 1.7s shows how content and images load incrementally, highlighting the impact of rendering delays." width="750" height="223">
<figcaption>A regular, progressive render (top) versus an anti-flicker
big-reveal (bottom). Which do you think is the better experience?</figcaption>
</figure>

`blocking=render` leaves the browser to its usual rendering process, so we can
still get a progressive render of the page, only now we do it in a way more
akin to loading a CSS file.

Finally, and this is counter to my own preferences and beliefs as a performance
engineer, we still risk leaking the experiment to the user when using an
anti-flicker snippet. Knowingly hiding a page for up to four seconds feels like
insanity to me, but at least we do have a timeout. The problem with anti-flicker
snippets is that if that four-second timeout is reached, we’ll still display the
page even if experiments haven’t completed—the `4000`ms is a magic number that
we use to hopefully win a race condition.

By using `blocking=render`, that timeout now becomes [governed by the browser’s
own heuristics](https://github.com/whatwg/html/issues/7131#footnote2), which is
almost definitely going to be longer than four seconds. While that does terrify
me, it does guarantee we don’t paint anything _too_ soon. No more race
conditions, but a potentially longer render-blocked period.

As I said at the top of the article, most of us won’t need `blocking=render`,
and those of us who do will know that we do.

## tl;dr

One handy takeaway is that, at present, `blocking=render` would cause any of the
following:

* `<script src async></script>`
* `<script src defer></script>`
* `<script src type=module></script>`
* `<script type=module>...</script>`

…to behave like this:

* `<link rel=stylesheet href>`

- - -

[^1]: [x.com/kurtextrem/status/1823698340388782591](https://x.com/kurtextrem/status/1823698340388782591)