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February 15, 2021

Filed under: tech»web

Between Amber and Chaos

There isn't, in my opinion, a cooler name for a web standard than the Shadow DOM. The closest runner-up is probably the SubtleCrypto API, and after a decade of Bitcoin the appeal of anything with "crypto" in the name is pretty cloudy. So it's a low bar, but still: Shadow DOM. Pretty cool name.

Although I've been using web components for a long time, I've only been using Shadow DOM with it for a couple of years, in generally in pretty limited ways. For an upcoming project at NPR, I took the chance to really dig into how it's used in a mixed-content environment, one where custom elements are not just leaves of the HTML tree, but also wrap branches of extensive HTML content. The experience was pretty eye-opening, and surprisingly positive!

Walking the pattern

Let's start by talking about what what it is. Like most of the tech under the web components "brand," Shadow DOM is meant to retroactively give developers tools that "explain" what the browser already does, and hook into the same extension points. The goal is to make it possible for regular people to rapidly build out new functionality, because there's no "magic" behind the scenes.

For example, let's create a humble <select> tag:

Right off the bat, this tag has some special treatment that we can't immediately explain through regular HTML: it has a "thumb" (the arrow on the right) that doesn't appear in the DOM and can't be meaningfully styled, but is clearly a UI element that reacts to events. The options, defined as children of the tag, are still surfaced visibly, but not in the same way that children of a paragraph are or a regular text element are. Instead, they're moved to a new location in the dropdown menu and shown conditionally (or, on mobile, through an entirely different UI context).

Using our previous HTML/JS toolkit, it's not possible to duplicate these behaviors, or similar behaviors from tags like <video> or <input type="range">. To explain the "magic" of these elements we need to add Shadow DOM. It gives developers an API to attach a hidden document fragment called a "shadow root" to any given element, which replaces the visible contents of the element. However, even though they're shown to us in the browser, the contents of that document fragment are hidden from normal JavaScript queries, and its CSS styles are isolated — from the inside, you have a blank slate to work from, and from the outside it's as though that shadow content is an intrinsic part of the tag itself, just like the select box's dropdown UI.

What about those select box options, which are written as child tags but appear in a very different way? For that, we add in a <slot> element: inside the shadow, this element will re-parent any children placed in the host element. For example, given a shadow-dom element with the following in its shadow root: <b> SHADOW START </b> <slot></slot> <b> SHADOW END </b>

We could write this in our page as: <shadow-dom> <i>HELLO WORLD</i> </shadow-dom>

The contents of the <i> element aren't shown directly. Instead, they're moved inside the slot element, meaning that the page output will read SHADOW START HELLO WORLD SHADOW END. But, and this is the cool part, that italic tag appears to scripts and dev tools as though it was just a regular child of the <shadow-dom> element — it can be styled as normal, you can query for it, attach event listeners, and edit it as normal. The bold tags, meanwhile, remain in the shadow: they're visible on the page, but they can't be accessed from scripts and their styles are completely isolated.

This, then, is how Shadow DOM "explains" how a select box works. The box itself, including the current item and the thumb UI, live in the shadow. The options you write into the tag are reparented to a slot inside the drop-down area, to be shown when you click the element. We can use this API to create self-contained UI for an application or document without having to worry about new markup or styles polluting the page.

Enter the Logrus

Not everything is rosy, of course. One long-standing complication is that custom elements can't touch their own contents or attributes during construction, for reasons that are tedious and not worth going into here, but they can attach and modify their shadow root. So it's really tempting in custom elements to do everything in a shadow, because it radically simplifies your templating. Now you have null problems. In Radio, I built the entire UI this way, which worked great until I needed to inspect an element that's inside three nested shadow roots, or if I needed to query for the current active element.

Another misunderstanding has been people thinking shadow roots can replace something like Styled Components in terms of style isolation. But Shadow DOM is more like an iframe than anything else: explicitly inherited style properties (like font family) will travel through, but otherwise it's a pretty hard barrier. If you want to provide styling hooks for a component, you need either provide preset options or document a set of CSS custom properties. More importantly, the mechanisms for injecting styles into a shadow root (typically by putting a <style> tag inside) don't play well with standard build tooling.

By contrast, actually populating Shadow DOM tends to be cumbersome without build tooling in place to help. A lot of tutorials recommend building it from an inert <template> tag, which used to be elegantly handled via HTML imports. Now that those are deprecated, you either have to place the Shadow DOM template in your page manually (no), lean into async component definition (awkward), embed the markup into your script as a big JS literal (ugly), or use a build plugin to pull strings in as needed (sigh). None of these are unworkable, or even that difficult, but none of them are nearly as nice as simply being able to define a component's styles, shadow markup, and behavior in a single, imported HTML file.

Major Arcana

My personal feeling is that the biggest barrier to effective Shadow DOM usage, in a lot of cases, is that many developers haven't learned about the browser as much as they've learned about React or another framework, and those frameworks have often diverged in philosophy from the DOM. If you're used to thinking of the page as a JSX function value, the idea of a secret, stateful document fragment that replaces the DOM you tried to render is probably pretty bizarre.

But as someone who writes a lot of minimalist code directly against browser APIs, I actually think Shadow DOM fits in well with my mental model of how elements work, and it has clarified a lot of my thinking on how to build effectively with custom elements — especially through slots and slotted elements.

I'm still learning and experimenting, but I feel comfortable saying that if you're building custom elements, the rule of thumb should be "use Shadow DOM, but not very much." The more you're able to expose HTML to the light DOM by surfacing it through slots, the easier it is to compose them and style content. For example, a custom element that creates a tabbed UI from its children is a great Shadow DOM use case: the tab list lives in the shadow and is generated implicitly by iterating over the slotted elements. Since the actual tab contents are placed back in the light DOM, they're still easy to style and inspect. To really go with the grain of the platform, the host component might show or hide those slotted blocks using the hidden attribute, instead of setting styles or adding classes.

The exception is for elements that should not have children (like input tags) or where children are used for configuration — think video tags or my old Leaflet map component. With these "leaf" components, Shadow DOM lets you treat inner HTML as a domain-specific language, while your visible content lives entirely in the shadow root. That's a great way to create customized behavior, but expose it to designers or novice front-end developers who are very comfortable with markup but would balk at writing a lot of JS.

Ultimately, Shadow DOM feels like it really crystallizes the role of custom elements as a tool for implementing UI widgets, not as a competitor for Svelte. Indeed, by providing a mechanism for moving complex functionality into an opaque facade, it's probably the biggest gift to the "web pages are for documents, not apps" crowd in several years: if you want to build a big single page app, Shadow DOM doesn't really move the needle, but it's great for injecting discrete units of content into an article. As someone who crosses that app/document divide a lot, I'm really excited to see what I can do with it this year.

Future - Present