When looking at Google’s PageSpeed Insights tool as part of the
compression work I noticed a suggestion that we preload our font files.
The tool suggests this should save about 300ms on first page load time.
***
Our font files are referenced from our CSS. This means that the browser
has to download and parse the CSS before it knows where to find the font
files. This means the requests happen in sequence.
We can make the requests happen in parallel by using a `<link>` tag with
`rel=preload`. This tells the browser to start downloading the fonts
before it’s even started downloading the CSS (the CSS will be the next
thing to start downloading, since it’s the next `<link>` element in the
head of the HTML).
Downloading fonts before things like images is important because once
the font is downloaded it causes the layout to repaint, and shift
everything around. So the page doesn’t feel stable until after the fonts
have loaded.
Google call this [cumulative layout shift](https://web.dev/cls/) which
is a score for how much the page moves around. A lower score means a
better experience (and, less importantly for us, means the page might
rank higher in search results)
We’re only preloading the WOFF2 fonts because only modern browsers
support preload, and these browsers also all support WOFF2.
We set an empty `crossorigin` attribute (which means anonymous-mode)
because the preload request needs to match the origin’s CORS mode. See
https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content#CORS-enabled_fetches
for more details.
We set `as=font` because this helps the browser use the correct content
security policy, and prioritise which requests to make first.
Done using isort[1], with the following command:
```
isort -rc ./app ./tests
```
Adds linting to the `run_tests.sh` script to stop badly-sorted imports
getting re-introduced.
Chosen style is ‘Vertical Hanging Indent’ with trailing commas, because
I think it gives the cleanest diffs, eg:
```
from third_party import (
lib1,
lib2,
lib3,
lib4,
)
```
1. https://pypi.python.org/pypi/isort
https://www.pivotaltracker.com/story/show/113448149
This commit adds a query string to assets URLs which is generated from a hash
of the file contents. When asset files are changed they will now be served from
a different URL, which means they wont be loaded from browser cache.
This is similar to how GOV.UK template adds its version number as a querystring
parameter for its assets.
This is mostly copied from Digital Marketplace utils:
https://github.com/alphagov/digitalmarketplace-utils/pull/102
They have it in a shared codebase, we only have one frontend app so don’t need
to do that.
Usage in a template:
``` jinja
{{ asset_fingerprinter.get_url('stylesheets/application.css') }}
```
Output:
```
static/stylesheets/application.css?418e6f4a6cdf1142e45c072ed3e1c90a
```
