11 June, 2018

Written by Harry Roberts on CSS Wizardry.

This year, I’ve been working closely with the wonderful
Coingaming team out in beautiful
Tallinn. We’ve
been working pretty hard on making their suite of online products much
faster, and I’ve
been the technical consultant leading the project. It’s been an incredibly fun
and rewarding engagement, and we’ve made some real business- and customer-facing
improvements. One of the key reasons I’ve found the project so fun is that it’s
given me a real chance to get very forensic. Naturally, a team should always
tackle the low-hanging fruit first, but once that’s done, you get to delve much
deeper into the weeds. This blog post comes from there.

Background

In order to get better control over how a series of thumbnail images are
displayed on the homepage of one of their products, the team opted to build them
not as elements, but as CSS background-images: this allowed them to
better size and position the images in their container, and, putting aside any
semantic concerns, it made sense from a styling point of view. My only
reservation was knowing that, because the images are defined in CSS rather than
HTML, their outgoing requests won’t start until the browser has created the
Render Tree: it’s not until all the CSS has been downloaded, parsed, and the
CSSOM constructed that the browser actually knows This div with this class
is currently visible on the page and its background is set to be this image: I’d
better download it!.

The waterfall below shows the browser waiting for CSSOM completion before it
dispatches any requests for any images—you can clearly see that the CSS needs to
finish before any images start. This is down to the simple fact that the browser
doesn’t know which (if any) images it will need until the CSSOM has been built:

This is too late for such important content of theirs—users want to see the
thumbnails as soon as possible.

By moving the images to elements (which is also semantically more
appropriate), the browser can discover them far sooner—as they become exposed to
the browser’s preload scanner—and dispatch their requests before (or in parallel
to) CSSOM completion:

This is stuff we already knew:

1. Browsers can’t possibly download background-images until they’ve built the
   CSSOM.
2. Browsers shouldn’t base—thus delay—the downloading of s on CSSOM
   completion. More on this later…

Where it gets interesting is when I started to wonder how different browsers
handle different types of image when they’re visible or not: an
element with display: none; applied to it ideally wouldn’t get downloaded, but
the only way the browser would know that the image is indeed hidden is if it
waits for CSSOM completion, thus slowing down the default behavior of the : what’s going to happen?

background-image

I will start with background-image as that’s what my client’s initial use case
was. I feel like the behaviour for background-image should be the most
predictable as there are certain impossibilities at play (e.g. we can’t download
a background-image until we’ve built the CSSOM).

What I Would Expect

I have a couple of expectations (or hopes) that I predict:

1. Browsers do not (can not) trigger a download for a background-image until
   it knows it needs it (i.e. until the CSSOM has been constructed).
2. Browsers would not download background-image that is applied to an element
   that is hidden from view (e.g. display: none;).

What Actually Happens

Chrome (v67.0.3396.79)

• Expectedly Chrome does not download any background-image until the CSSOM
  has been constructed:

  

• Unexpectedly, Chrome will download a background-image that isn’t visible
  to the user:

  

Safari (v11.1 (13605.1.33.1.4))

• Expectedly Safari does not download any background-image until the CSSOM
  has been constructed:

  

• Expectedly, Safari will not download a background-image that isn’t
  visible to the user:

  

Firefox (v60.0.1)

• Expectedly Firefox does not download any background-image until the
  CSSOM has been constructed:

  

• Expectedly, Firefox will not download a background-image that isn’t
  visible to the user:

  

Opera (v53.0.2907.68)

• Expectedly Opera does not download any background-image until the CSSOM
  has been constructed:

  

• Unexpectedly, Opera will download a background-image that isn’t visible
  to the user:

  

Edge (v17.17134)

• Expectedly Edge does not download any background-image until the CSSOM
  has been constructed.

  

• Unexpectedly, Edge will download a background-image that isn’t visible
  to the user.

  

Summary

N.B. Yes or No represents factual information. ✓ and ✗
represent what I would consider good/expected and bad/unexpected behaviour,
respectively.

Verdict

Firefox and Safari seem to have the most preferred behaviour here: they
won’t download background-images that they know they won’t need.

Chrome, Opera, and Edge will all download background-images that
are applied to invisible elements. This feels wasteful, but I suspect it is
a preemptive optimisation to ensure that the image is on the client before the
potential event that the element becomes visible. I feel that—if this is the
case—this is an optimisation that should be left to the developer.

Next, let’s take a look at how browsers handle s.

What I Would Expect

1. Browsers will download completely independently of CSSOM
   construction. Blocking on CSSOM construction seems very
   inefficient, and would lead to delays in downloading content.
2. Accordingly, browsers will download that end up being hidden from
   view, i.e. display: none;. If the browser starts downloading
   before CSSOM construction (as I expect) then it has no way of knowing yet
   whether that image is needed or not.

What Actually Happens

Chrome

Safari

Firefox

Opera

Edge

Summary

N.B. Yes or No represents factual information. ✓ and ✗
represent what I would consider good/expected and bad/unexpected behaviour,
respectively.

Verdict

Firefox appears to block on CSSOM construction. This seems like
a bad idea—no s will begin downloading until Firefox has downloaded,
parsed, and applied the CSS. This means that if you have blocking stylesheets,
they’re blocking your . This would be particularly troublesome if are key content (think Imgur, Flickr, etc.).

Firefox gets weirder still! It waits until it’s constructed the CSSOM before
it fires off any requests, which means it knows if the will
be visible or not, but if the is invisible, it will download it
anyway. This is a double-hit: Firefox blocks on CSSOM construction yet
still downloads that aren’t visible.

Key Takeaways

The Facts

• Chrome, Opera, and Edge will download background-images that
  aren’t required for first render. This means that hidden DOM nodes that have
  a background-image applied to them will still have that background-image
  downloaded. Beware unexpected downloads.
• Firefox will block downloads on CSSOM construction, meaning
  later-than-expected downloads. Beware delays.
• Further, Firefox will still download the even if it wasn’t
  needed. Beware unexpected downloads.

How Might this Affect You?

If you’re a product that relies heavily on content imagery (e.g. Flickr, online
publication, photographer) then Firefox will not download any of those images
until it’s dealt with your CSS. You should look into preloading any key image
content.

If you’re making heavy use of background images and, for whatever reason, are
not showing all of them for first render, beware that some browsers will go
ahead and download them anyway: you might want to look into better strategies
for hidden content (e.g. removal from the DOM rather than display: none;).