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Browser Support Heuristics

In web development it’s useful when we can say “if the browser supports X, then we know it also supports Y”.

There was a small lightbulb moment at work earlier this year when we worked out that:

if the user’s browser supports CSS Grid, then you know you it also supports custom properties.

Knowing this means that if you wrap some CSS in an @supports(display:grid) then you can also safely use custom properties within that block.

I love this rule of thumb! It saves you looking up for each feature and comparing the browser support.

This weekend I did some unplanned rabbit-holing on the current state of (and best practices for using) ES modules in the browser, as-is and untranspiled. That revealed another interesting rule of thumb:

any browser that supports <script type="module"> also supports let and constasync/await, the spread operator, etc.

One implication of this is that if you currently build a large JavaScript bundle (due to being transpiled down to ES 3/5 and including lots of polyfills) and ship this to all browsers including the modern ones… you could instead improve performance for the majority of your visitors by configuring your bundler to generate two bundles from your code then doing:

// only one of these will be used. 
<script type="module" src="lean-and-modern.js"></script>
<script nomodule src="bulky-alternative-for-old-browsers.js"></script>

I might make a little page or microsite for these rules of thumb. They’re pretty handy!

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