Exploring Fitness

The mobile-first design methodology is great—it focuses on what really matters to the user, it’s well-practiced, and it’s been a common design pattern for years. So developing your CSS mobile-first should also be great, too…right? 

Well, not necessarily. Classic mobile-first CSS development is based on the principle of overwriting style declarations: you begin your CSS with default style declarations, and overwrite and/or add new styles as you add breakpoints with min-width media queries for larger viewports (for a good overview see “What is Mobile First CSS and Why Does It Rock?”). But all those exceptions create complexity and inefficiency, which in turn can lead to an increased testing effort and a code base that’s harder to maintain. Admit it—how many of us willingly want that?

On your own projects, mobile-first CSS may yet be the best tool for the job, but first you need to evaluate just how appropriate it is in light of the visual design and user interactions you’re working on. To help you get started, here’s how I go about tackling the factors you need to watch for, and I’ll discuss some alternate solutions if mobile-first doesn’t seem to suit your project.

Advantages of mobile-first

Some of the things to like with mobile-first CSS development—and why it’s been the de facto development methodology for so long—make a lot of sense:

Development hierarchy. One thing you undoubtedly get from mobile-first is a nice development hierarchy—you just focus on the mobile view and get developing. 

Tried and tested. It’s a tried and tested methodology that’s worked for years for a reason: it solves a problem really well.

Prioritizes the mobile view. The mobile view is the simplest and arguably the most important, as it encompasses all the key user journeys, and often accounts for a higher proportion of user visits (depending on the project). 

Prevents desktop-centric development. As development is done using desktop computers, it can be tempting to initially focus on the desktop view. But thinking about mobile from the start prevents us from getting stuck later on; no one wants to spend their time retrofitting a desktop-centric site to work on mobile devices!

Disadvantages of mobile-first

Setting style declarations and then overwriting them at higher breakpoints can lead to undesirable ramifications:

More complexity. The farther up the breakpoint hierarchy you go, the more unnecessary code you inherit from lower breakpoints. 

Higher CSS specificity. Styles that have been reverted to their browser default value in a class name declaration now have a higher specificity. This can be a headache on large projects when you want to keep the CSS selectors as simple as possible.

Requires more regression testing. Changes to the CSS at a lower view (like adding a new style) requires all higher breakpoints to be regression tested.

The browser can’t prioritize CSS downloads. At wider breakpoints, classic mobile-first min-width media queries don’t leverage the browser’s capability to download CSS files in priority order.

The problem of property value overrides

There is nothing inherently wrong with overwriting values; CSS was designed to do just that. Still, inheriting incorrect values is unhelpful and can be burdensome and inefficient. It can also lead to increased style specificity when you have to overwrite styles to reset them back to their defaults, something that may cause issues later on, especially if you are using a combination of bespoke CSS and utility classes. We won’t be able to use a utility class for a style that has been reset with a higher specificity.

With this in mind, I’m developing CSS with a focus on the default values much more these days. Since there’s no specific order, and no chains of specific values to keep track of, this frees me to develop breakpoints simultaneously. I concentrate on finding common styles and isolating the specific exceptions in closed media query ranges (that is, any range with a max-width set). 

This approach opens up some opportunities, as you can look at each breakpoint as a clean slate. If a component’s layout looks like it should be based on Flexbox at all breakpoints, it’s fine and can be coded in the default style sheet. But if it looks like Grid would be much better for large screens and Flexbox for mobile, these can both be done entirely independently when the CSS is put into closed media query ranges. Also, developing simultaneously requires you to have a good understanding of any given component in all breakpoints up front. This can help surface issues in the design earlier in the development process. We don’t want to get stuck down a