The Fourth Way to Inject CSS

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Last week, Jeremy Keith wrote an excellent summary of the three ways to inject CSS into a web document. In short, he said:

There are three ways—that I know of—to associate styles with markup.

External CSS

<link rel="stylesheet" href="/path/to/styles.css">

Embedded CSS

<style>element { property: value; }</style>

Inline CSS

<element style="property: value"></element>

While talking about external CSS, he hinted at what I consider to be a distinct fourth way with its own unique use cases:; using the Link: HTTP header. I’d like to share with you how it works and why I think it needs to be kept in people’s minds, even if it’s not suitable for widespread deployment today.

Injecting CSS using the Link: HTTP Header

Every one of Jeremy’s suggestions involve adding markup to the HTML document itself. Which makes sense; you almost always want to associate styles with a document regardless of the location it’s stored or the medium over which it’s transmitted. The most popular approach to adding CSS to a page uses the <link> HTML element, but did you know… the <link> element has a semantically-equivalent HTTP header, Link:.

Bash shell running the command "curl -i https://hell.meiert.org/core/php/link.php". The response includes a HTTP Link: header referencing a CSS stylesheet; this is highlighted.
The only active example I’ve been able to find are test pages like Jens Oliver Meiert’s (pictured), Louis Lazaris’s , and Anne van Kesteren’s, but it’s possible that others are hiding elsewhere on the Web.

According to the specifications, the following HTTP responses are equivalent in terms of the CSS that would be loaded and applied to the document:

Traditional external CSS injection:

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8

<!doctype html>
<html>
  <head>
    <title>My page</title>
    <link rel="stylesheet" href="/style/main.css">
  </head>
  <body>
    <h1>My page</h1>
  </body>
</html>

Link: header CSS injection:

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Link: </style/main.css>; rel="stylesheet"

<!doctype html>
<html>
  <head>
    <title>My page</title>
  </head>
  <body>
    <h1>My page</h1>
  </body>
</html>
Diagram showing a web browser requesting a document from a web server, the web server finding the document and returning it after attaching HTTP headers.
A webserver adds headers when it serves a document anyway. Adding one more is no big deal.

Why is this important?

This isn’t something you should put on your website right now. This (21-year-old!) standard is still only really supported in Firefox and pre-Blink Opera, so you lose perhaps 95% of the Web (it could be argued that because CSS ought to be considered progressive enhancement, it’s tolerable so long as your HTML is properly-written).

If it were widely-supported, though, that would be a really good thing: HTTP headers beat meta/link tags for configurability, performance management, and separation of concerns. Need some specific examples? Sure: here’s what you could use HTTP stylesheet linking for:

Diagram showing a reverse proxy server modifying the headers set by an upstream web server in response to a request by a web browser.
You have no idea how many times in my career I’d have injected CSS Link: headers using a reverse proxy server the standard was universally-implemented. This technique would have made one of my final projects at the Bodleian so much easier…
  • Performance improvement using aggressively preloaded “top” stylesheets before the DOM parser even fires up.
  • Stylesheet injection by edge caches to provide regionalised/localised changes to brand identity.
  • Strong separation of content and design by hosting content and design elements in different systems.
  • Branding your staff intranet differently when it’s accessed from outside the network than inside it.
  • Rebranding proprietary services on your LAN without deep inspection, using reverse proxies.
  • Less-destructive user stylesheet injection by plugins etc. that doesn’t risk breaking icky on-page Javascript (e.g. theme switchers).
  • Browser detection? 😂 You could use this technique today to detect Firefox. But you absolutely shouldn’t; if you think you need browser detection in CSS, use this instead.

Unfortunately right now though, stylesheet Link: headers remain consigned to the bin of “cool stylesheet standards that we could probably use if it weren’t for fucking Google”; see also alternate stylesheets.

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