A Bit on Web Component Libraries

A run of Web Components news crossed my desk recently so I thought I’d group it up here.

To my mind, one of the best use cases for Web Components is pattern libraries. Instead of doing, say, <ul class="nav nav-tabs"> like you would do in Bootstrap or <div class="tabs"> like you would in Bulma, you would use a custom element, like <designsystem-tabs>.

The new Shoelace library uses the sl namespace for their components. It’s a whole pattern library entirely in Web Components. So the tabs there are <sl-tab-group> elements.

Why is that good? Well, for one thing, it brings a component model to the party. That means, if you’re working on a component, it has a template and a stylesheet that are co-located. Peeking under the hood of Shoelace, you can see this is all based on Stencil.

Another reason it’s good is that it means components can (and they do) use the Shadow DOM. This offers a form of isolation that comes right from the web platform. For CSS folks like us, that means the styling for a tab in the tab component is done with a .tab class (hey, wow, cool) but it is isolated in that component. Even with that generic of a name, I can’t accidentally mess with some other component on the page that uses that generic class, nor is some other outside CSS going to mess with the guts here. The Shadow DOM is a sort of wall of safety that prevents styles from leaking out or seeping in.

I just saw the FAST framework¹ too, which is also a set of components. It has tabs that are defined as <fast-tabs>. That reminds me of another thing I like about the Web Components as a pattern library approach: if feels like it’s API-driven, even starting with the name of the component itself, which is literally what you use in the HTML. The attributes on that element can be entirely made up. It seems the emerging standard is that you don’t even have to data-* prefix the attributes that you also make up to control the component. So, if I were to make a tabs component, it might be <chris-tabs active-tab="lunch" variation="rounded">.

Perhaps the biggest player using Web Components for a pattern library is Ionic. Their tabs are <ion-tabs>, and you can use them without involving any other framework (although they do support Angular, React, and Vue in addition to their own Stencil). Ionic has made lots of strides with this Web Components stuff, most recently supporting Shadow Parts. Here’s Brandy Carney explaining the encapsulation again:

Shadow DOM is useful for preventing styles from leaking out of components and unintentionally applying to other elements. For example, we assign a .button class to our ion-button component. If an Ionic Framework user were to set the class .button on one of their own elements, it would inherit the Ionic button styles in past versions of the framework. Since ion-button is now a Shadow Web Component, this is no longer a problem.

However, due to this encapsulation, styles aren’t able to bleed into inner elements of a Shadow component either. This means that if a Shadow component renders elements inside of its shadow tree, a user isn’t able to target the inner element with their CSS.

The encapsulation is a good thing, but indeed it does make styling “harder” (on purpose). There is an important CSS concept to know: CSS custom properties penetrate the Shadow DOM. However, it was decided — and I think rightly so — that “variablizing” every single thing in a design system is not a smart way forward. Instead, they give each bit of HTML inside the Shadow DOM a part, like <div part="icon">, which then gives gives the ability to “reach in from the outside” with CSS, like custom-component::part(icon) { }.

I think part-based styling hooks are mostly fine, and a smart way forward for pattern libraries like this, but I admit some part of it bugs me. The selectors don’t work how you’d expect. For example, you can’t conditionally select things. You also can’t select children or use the cascade. In other words, it’s just one-off, or like you’re reaching straight through a membrane with your hand. You can reach forward and either grab the thing or not, but you can’t do anything else at all.

Speaking of things that bug people, Andrea Giammarchi has a good point about the recent state of Web Components:

Every single library getting started, including mine, suggest we should import the library in order to define what [sic] supposed to be a “portable Custom Element”.

Google always suggests LitElement. Microsoft wants you to use FASTElement. Stencil has their own Component. hyperHTML has their own Component. Nobody is just using “raw” Web Components. It’s weird! What strikes me as the worst part about that is that Web Components are supposed to be this “native platform” thing meaning that we shouldn’t need to buy into some particular technology in order to use them. When we do, we’re just as locked to that as we would be if we just used React or whatever.

Andrea has some ideas in that article, including the use of some new and smaller library. I think what I’d like to see is a pattern library that just doesn’t use any library at all.

1. FAST calls itself a “interface system,” then a “UI framework” in consecutive sentences on the homepage. Shoelaces calls itself a “library” but I’m calling it a “pattern library.” I find “design system” to be the most commonly used term to describe the concept, but often used more broadly than a specific technology. FAST uses that term in the code itself for the wrapper element that controls the theme. I’d say the terminology around all this stuff is far from settled.

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