UA-Client-Hints and Progressive Enhancement

This morning I was reading an important reminder from Jeremy Keith. It’s about Progressive Enhancement works and how the server plays an important role in the enhancement step of the process

Even with better tools and abilities for client side work, we still need a server to produce some content and send it to the browser:

These days this is called “server-side rendering”, even though for decades the technical term was “serving a web page” (I’m pretty sure the rendering part happens in a browser).


Reading this article and at the same time thinking of the changes Chrome is making to the User-Agent and the introduction of “UA Client Hints”, makes me wonder how this will affect Progressive Enhancement as a mindset for creating universally accessible web applications.

I am a great fan of Progressive Enhancement simply because the mindset acknowledges that there might be a better tool for the job than JavaScript and other places better suited to execute logic than the browser.

My worries are:

Less Progressive Enhancement on the 1st request

Because the access to client hints is guarded by opt-in, there is only so much the server can do to optimize and adapt. Worst case we’re in a “better bring my cross country skis to summer vacation”-scenario, and let the JavaScript handle it.

UA-CH: designed by front end devs, for front end devs

…at least it feels like it when reading the explainer

1995 called…

It is true that some servers used to block certain browsers based on the contents of the User-Agent in the early days of the web. Who has experienced this lately? The business of server-side device detection has settled and is the foundation of countless use cases on the web, but not obvious. “Server Side Rendering” is only one use case. Advertising, fraud/spam/but detection, analytics are others.

If a browser (or feature) is blocked from a site today, it’s blocked by JavaScript. What happened to feature detection? Is it suddenly OK to implement functionality based on model, versions and platform? Of course it is, it has always been. Sometimes there are no way around. 

But the server must have the same abilities! As the spec stands now, the server is discriminated:

JavaScript API require no opt-in, and is available cross-domain!

Accessing the Client Hints through the JavaScript API does not require an opt-in the same way as the server accessible request headers does. Moreover, the JavaScript API is available cross domain with no mechanism to control which 3rd party get access. 

Spec says this behaviour is “accounted for”, but why wouldn’t you restrict Facebook or any other 3rd party to fingerprint your users if you could? I would. It’s naive to consider the presence of all 3rd parties included on a webpage as an opt-in.

The fingerprinting surface is much bigger, and impossible to control, with the JavaScript UA-CH API compared to the current situation where the navigator.User-Agent is the most “fingerprint-like” thing we have.

Active fingerprinting better than passive?

No. Besides, passive fingerprinting will always be there. The proposed JavaScript API just adds more entropy and improves the performance and accuracy of active fingerprinting.

Will UA-CH lie?

Yes. Of course. Just like the User-Agent. Common lies are better handled by server side device detection than all front end developers individually. 


Maybe server-side device detection used to be a problem, but this business has matured and settled. The proposed changes make it harder for the server to do content negotiation and play its role in Progressive Enhancement.

If device detection is bad, why give this opportunity to the font end devs? Sure, parsing the User-Agent is hard. Device detection products on the server side has figured out how to do it effectively, but what will happen when every front end developer will have to learn the pitfalls of device detection? It’s not about how hard the User-Agent is to parse, it’s about poor programming choices and lack of a Progressive Enhancement mindset. I envision similar scenarios to the good ol’ “this webpage only works with Internet Explorer”.

Do not use JavaScript for tasks related to recognising device or browser with the purpose of deciding what- and how to implement certain functionality. My $0.02

The situation we have now is the worst of both worlds: server-side rendering followed by a tsunami of hydration.

I fear that the UA-CH JavaScript API will become motivation for pouring more JavaScript at tasks better solved by Server Side Rendering (SSR) (or the server, as old people like me calls it). And the worst part, the server may not even be able to help out anymore!

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