The browser uses the `width` and `height` attributes of the image tag to
allocate space on the page for the image.
If these aren’t provided then the browser will assume the image takes up
no space, until it’s downloaded it and had a look at what the file’s
dimensions are. This causes the layout of the page to jump once the
image downloads.
`149 × 150px` is the native size of the image. But we don’t want it to
display at that size, so this commit also adds some extra CSS which
keeps it looking the same, namely:
- the full width of the 1/4 page column on desktop
- the full width of the column minus a `40px` gutter either side on
mobile (by using `box-sizing: border-box` the `40px` of padding is
subtracted from the 100% width, rather than added to it)
This hides the "Register" button and shows an error that's specific
to one of two ways a browser may not support WebAuthn:
- JavaScript is disabled (there's no possible fallback for this).
- WebAuthn API is not supported (e.g. on Internet Explorer).
We could add a similar check for the API in the JS code to handle
the button click, but hiding it seems like enough protection.
In order to avoid elements flashing when the page loads, this uses
a view macro to embed a script at the start of the body element,
which is the same approach used for the "js-enabled" class flag [1].
Tested with Chrome and IE 11.
[1]: https://github.com/alphagov/govuk-frontend/blob/main/src/govuk/template.njk#L31
