Heydon’s <watched-box> is a damn fantastic tool. It’s a custom element that essentially does container queries by way of class names that get added to the box based on size breakpoints that are calculated with ResizeObserver. It’s like a cleaner version of what Philip was talking about a few years ago.
I’m sure I’d be happy using <watched-box> on production, as it’s lightweight, has no dependencies, and has a straightforward approach.
For development, I had the idea of using Zach’s interesting little <resize-asaurus> web component. It wraps elements in another box that is resizeable via CSS and labels it with the current width. That way you don’t have to fiddle with the entire browser window to resize things — any given element becomes resizable. Again, just for development and testing reasons.
You’d wrap them together like…
<resize-asaurus>
<watched-box widthBreaks="320px, 600px">
<div class="card">
...
</div>
</watched-box>
</resize-asaurus>Which allows you to write CSS at breakpoints like…
.card {
.w-gt-320px & { }
.w-gt-600px & { }
}That’s surely not what the CSS syntax for container queries syntax will end up being, but it accomplishes the same thing with clear and understandable generated class names.
Example!
Live demo!