Natalie

One of the major thorns in my side when working on CSS crimes copost creation tools has been ensuring that they look good on everyone's screens. In normal CSS authoring, you'd use @media rules to provide "responsive" styles that adjust themselves based on the width of the user's viewport. Once a user's screen or window size goes below certain "breakpoints", elements rearrange themselves or change their sizing logic to accommodate. But because you can't write a top-level stylesheet in a copost, that won't work for us.

I've figured out a way to work around this, though. CSS Values and Units Level 3 introduced the vw unit, which is a length that measures exactly 1% of the size of the user's viewport. Using this and some clever math, we can create inline breakpoints (with a few substantial caveats). For example, let's take the breakpoint 768px from Cohost's own CSS. Most tablets and desktops are wider than 768px, while phones (in vertical orientation) are universally narrower, so it's nice to selecting for desktop versus mobile users. We can then write our inline breakpoint like this:

min(max((100vw - 768.1px)*9999, <mobile value>), <desktop value>)

This expression will essentially return <mobile value> on mobile devices and <desktop value> on desktop devices. Great! But how does it work? Let's break it down:

• Since 1vw is 1% of the viewport width, 100vw is the width of the entire viewport.
• 100vw - 768.1px is a positive number if the viewport is larger than 768px, and negative if it's smaller or equal to 768px. (We add the .1 to ensure that we don't get weird behavior for exactly-768px-wide viewports.)
• We multiply this by 9999 to get a very large negative number for mobile devices and positive number for desktop devices. (Note that if this is five 9s Firefox shits the bed for some reason, so: exactly four 9s.)
• Finally, we use max() and min() to convert this to the actual numbers we want, since <mobile value> will always be larger than the very negative number and <desktop value> will always be smaller than the very positive number.

As I mentioned, there are a few caveats:

• This only works for numeric values. You can't use this to set an element to display: none on mobile, for example. To make the demo above, I had to set the width to 0px instead.

• It specifically only works for lengths. Calculation expressions like min() and max() have to have comparable units, so you can't use this to set other types of value like opacity or timing.

• As written <mobile value> must be less than <desktop value>, but you can fix this by changing the order of the subtraction expression.

I plan to use this to update my grid generator to be more intelligent about font choice selection pretty soon, and I hope other people will find it useful as well.