Choosing between min-width and max-width media queries

There's a decent number of designers/developers that still think of "the desktop" as their primary design manifestation, with other (generally smaller) sizes as secondary; often these appear as afterthoughts, since there can be significant visual clues as to the amount of effort that has gone into these other screen sizes when compared to their desktop counterparts. Often, when one examines the CSS of these sites, `max-width` is the media feature of choice. This makes sense. If a design is built desktop-first, then all the CSS for the desktop-ish version of the design has been written, and must be overridden with more CSS for any other breakpoints. If a given desktop-ish width is our baseline and we are unwilling or unable to refactor our CSS, then it seems logical to override what is now our base CSS for all viewport widths with those we only want to apply to smaller widths. This can lead to a situation I've often joked about: writing styles only to undo them later. This is an example I often use (assume .related takes the form of a sidebar):

.content {
  width: 60%;
}

.related {
  width: 40%;
}

@media screen and (max-width: 37.4em) {
  .content,
  .related {
    width: 100%;
  }
}

The above example omits a lot, but I'm sure you get the idea. This approach, when used over a bunch of components, can greatly increase the amount of CSS needed to complete the project. But the main problem is how illogical it is, because when embracing the fact the block-level elements are 100% of their parent by default, then the following makes much more sense:

@media screen and (min-width: 37.5em) {
  .content {
    width: 60%;
  }

  .related {
    width: 40%;
  }
}

Here, we're making use of the default state of block-level elements, and overriding them when they need to change from the default. Again, it might seem clear to some readers, but it doesn't take looking at the source of many websites before you understand why I'm clarifying it here.

There are a couple of reasons why, even when you know better, you might deliberately choose to use an approach similar to max-width approach above:

1. You're a developer and you received desktop-only, desktop-first, or other-sizes-as-an-afterthought comps from the designer. How can you know? If you've only designed or received designs for desktop-ish widths, this is what you're dealing with. If you've got nice detailed desktop-ish designs with a few "mobile" or "tablet" examples thrown in there, this is what you're dealing with. In these cases, may the max-width be with you, unless you have a budget and/or schedule that allows you to refactor during development.
2. You're making an existing website responsive, and the existing CSS is design baggage you've inherited and cannot change, for whatever reason.
3. You're compensating for the fact that we don't currently have element queries, and you're handling this with CSS instead of using a JavaScript polyfill for a non-existent spec.

Here's my clarification of "it depends" for this particular question. And remember it's just my own opinion based on my own experiences: look at default display of a given element. If you have to override this default for smaller screens, use max-width. If the default is usable on small screens, use min-width, and only when the element needs to deviate from the default. And of course, I recommend letting the content determine where that happens. A good example of using max-width to override an element's default display so that it works better on smaller screens is when using tables. Imagine a table that contains too much content to display in its default form on small screens. One approach might be:

@media only screen and (max-width: 30em) {
  .big-table tr,
  .big-table td {
    display: block;
  }
}

This turns each row (and cell) into a block on narrow screens. The table can become quite long vertically and more styling is usually required, but it's often better than the back-and-forth horizontal scrolling that would otherwise be required to read the content. In this case, it makes all the sense in the world to leave the table in default form, except for in browsers that understand the media query and where the screen is no larger than, in this case, 30em. Other elements (I would venture to say most other elements) that have defaults that work fine on smaller screens, only need changing when necessary on larger screens; using min-width is then a solid choice. In short: let element defaults help determine which media feature to use in your media queries. Thanks to Brad Frost for reviewing this. He also has a post with related topics.