Approach

Summary

We'll dive into each of these more thoroughly and at a high level; here's how our approach works.

• Components should be responsive and mobile-first.
• Components should be built with a base class and extended via modifier classes.
• Component states should obey a common z-index scale.
• Whenever possible, am HTML and CSS implementation over JavaScript should be preferred.
• Whenever possible, utilities over custom styles should be used.
• Whenever possible, enforcing strict HTML requirements (children selectors) should be avoided.

Responsive

Front's responsive styles are built to be responsive, an approach that's often referred to as mobile-first. We use this term in our docs and largely agree with it, but at times it can be too broad. While not every component must be entirely responsive in Front, this responsive approach is about reducing CSS overrides by pushing you to add styles as the viewport becomes larger.

Across Front, you'll see this most clearly in our media queries. In most cases, we use min-width queries that begin to apply at a specific breakpoint and carry up through the higher breakpoints. For example, a .d-none applies from min-width: 0 to infinity. On the other hand, a .d-md-none applies from the medium breakpoint and up.

At times we'll use max-width when a component's inherent complexity requires it. At times, these overrides are functionally and mentally clearer to implement and support than rewriting core functionality from our components. We strive to limit this approach, but will use it from time to time.

Classes

Aside from our Reboot, a cross-browser normalization stylesheet, all our styles aim to use classes as selectors. It means steering a clear of type selectors (e.g., input[type="text"]) and extraneous parent classes (e.g., .parent .child) that make styles too specific to easily override.

As such components should be built with a base class that houses common, not-to-be overridden property-value pairs. For example, .btn and .btn-primary. We use .btn for all the common styles like display, padding, and border-width. We then use modifiers like .btn-primary to add the color, background-color, border-color and etc.

Modifier classes should only be used when there are multiple properties or values to be changed across multiple variants. Modifiers are not always necessary, so be sure you're actually saving lines of code and preventing unnecessary overrides when creating them. Good examples of modifiers are our theme color classes and size variants.

z-index scales

There are two z-index scales in Front—elements within a component and overlay components.

Component elements

• Some components in Front are built with overlapping elements to prevent double borders without modifying the border property. For example, button groups, input groups, and pagination.
• These components share a standard z-index scale of 0 through 3.
• 0 is default (initial), 1 is :hover, 2 is :active/.active, and , 3 is :focus.
• This approach meets our expectations of highest user priority. If an element is focused, it's in view and at the user's attention. Active elements are second highest because they indicate state. Hover is third highest because it indicates user intent, but nearly anything can be hovered.

Overlay components

Front includes several components that function as an overlay of some kind. This includes, in order of highest z-index, dropdowns, fixed and sticky navbars, modals, tooltips, and popovers. These components have their own z-index scale that begins at 1000. This starting number is random and serves as a small buffer between our styles and your project's custom styles.

Each overlay component increases its z-index value slightly in such a way that common UI principles allow user focused or hovered elements to remain in view at all times. For example, a modal is document blocking (e.g., you cannot take any other action save for the modal's action), so we put that above our navbars.

HTML and CSS over JS

Whenever possible, we prefer to write HTML and CSS over JavaScript. In general, HTML and CSS are more prolific and accessible to more people of all different experience levels. HTML and CSS are also faster in your browser than JavaScript, and your browser generally provides a great deal of functionality for you.

This principle is our first-class JavaScript API which are data attributes. You don't need to write nearly any JavaScript to use our JavaScript plugins; instead, write HTML.

Lastly, our styles build on the fundamental behaviors of common web elements. Whenever possible, we prefer to use what the browser provides. For example, you can put a .btn class on nearly any element, but most elements don't provide any semantic value or browser functionality. So instead, we use <button>s and <a>s.

Utilities

Utility classes are a powerful ally in combatting CSS bloat and poor page performance. A utility class is typically a single, immutable property-value pairing expressed as a class (e.g., .d-block represents display: block;). Their primary appeal is speed of use while writing HTML and limiting the amount of custom CSS you have to write.

Specifically regarding custom CSS, utilities can help combat increasing file size by reducing your most commonly repeated property-value pairs into single classes. This can have a dramatic effect on your projects.

Flexible HTML

While not always possible, we strive to avoid being overly dogmatic in our HTML requirements for components. Thus, we focus on single classes in our CSS selectors and try to avoid immediate children selectors (>). This gives you more flexibility in your implementation and helps keep our CSS simpler and less specific.