The government doesn't design roads to conform to the top brands of cars and trucks. It specs them to a standard and any manufacturer can have their product certified as compliant. This both protects the public and gives anyone an opportunity to provide products no matter how small their market share is. Doing it the other way around would be madness, and I don't see why the same principle shouldn't apply here.
This is exactly why sites should be tested for their conformance with the standard rather than with specific browsers. Testing against specific browsers just encourages browser-makers to continue to avoid fixing their shit.
When people complain that they can't submit their DMV forms (or whatever) and you say, "well we followed all the standards, go find another browser" and they say "which one" and you say "I don't know, we didn't test with any browsers, we just test against the standard" who do you think they'll blame?
Is it really a standard if the dominant vendor just ignores the standard and does what it wants because then it becomes the "actual standard" ?
As a (terrible) example, look at FIPS. The government has the power to mandate a standard that everyone needs to implement. If instead of "supporting specific browser vendors" they "support a specific standard" then all vendors have a target that works with an agreed upon common ground.
Plenty of standards are guidelines. The point is the website’s purpose trumps dogmatic adherence to a standard. If the site works against standards but not in the browser, it’s a failure.
Basically, no proprietary features whatsoever. No draft features. No optionally-supported standard features without a fallback that fully covers the same underlying use case.
This would be beneficial guidance for browser developers, but also for anyone developing government sites. Saying "whatever chrome is doing is our standard" is a cop out.
If you want test that you are adhering to a standard (if there is no testsuite available) you need to test against more than 2 independent implementations.
USDS serves basic pages and applets which work fine on Firefox. It is only a handful of very complex web apps like Photopea where the developer will say "run this on Chrome for best results".
The most recent example I've run into is Snapchat's web client: It reject's Firefox purely by user agent string, but then works perfectly in Firefox if you just have your browser lie.
Speaking of Google, won't this interfere with their "look we're not a monopoly" payments to Mozilla Corp?
They really should be hiring a pollster to ask people which browser they use. It might be a bit difficult for those who use the default, but it wouldn't be that difficult to ask a couple of followup questions (e.g. which device) to determine which default browser that is.
On top of that, said lack of knowledge is a stated goal of all privacy-focused browsers. GA-blindness is an implementation detail of these policies. Any method the government could use to accurately track that information is effectively a bug in need of fixing from the POV of the browser's developers.
The most practical answer is, as was posted by someone else here, the government spec'ing to a standard instead of a set of browsers, which it really should be doing in the first place. The mere fact that the current setup means that the government accidentally makes and/or breaks winners in that space is justification enough.