TL;DR: hey check out this weird CodePen I made
So at work, I had the opportunity to write a small tool that dumps the current state of the document into a single static HTML file, making no network requests. For the most part, it turned out to be a lot simpler than I expected! DOM APIs are really good nowadays (all hail querySelectorAll). It should have taken me like 3 hours tops, from starting up the editor to pushing the PR, but instead it took all day, because of one very weird bug.
Virtual StyleSheets
One thing I learned poking around in our app for a bit is that, not all <style> elements have to be present in the document! That’s right, you can define CSS rules entirely in JavaScript, never having their state show up in the DOM at all. You do this thru the CSSStyleSheet and document.adoptedStyleSheets APIs, supported on all major browsers:
const styleSheet = new CSSStyleSheet();
styleSheet.insertRule("* { color: red !important; }");
document.adoptedStyleSheets.push(styleSheet);
This example code will turn all text on the page red. Try it out in your browser console now!!
In any case, I knew about this restriction beforehand, and was prepared. The document.styleSheets objects can also be modified in-place, so really the only way to properly dump everything was to run something like this:
// Create a new empty document:
// <html><head><style></style></head></html>
const outputDocument = new Document();
const html = outputDocument.createElement("html");
outputDocument.appendChild(html);
const head = outputDocument.createElement("head");
html.appendChild(head);
const style = outputDocument.createElement("style");
head.appendChild(style);
// Add all rules to the style element
const rules = [...document.styleSheets, ...document.adoptedStyleSheets].flatMap(
(styleSheet) => [...styleSheet.cssRules].map((cssRule) => cssRule.cssText),
);
style.sheet.replaceSync(rules.join("\n"));
Virtual Documents
As it turns out, this doesn’t work! Why?? As it turns out, browsers are very finicky about element state; many properties are defined only when they are actively being rendered, which this virtually-created Document is not. This extends for everything created in the “context”1 of that virtual Document too, so style.sheet will be undefined in the above example.
No problem, though! Instead of modifying thru style.sheet.replaceSync, we can do what MDN recommends on another page somewhere else, and use innerHTML:
style.innerHTML = rules.join("\n");
and boom, everything is working! Except wait a minute, certain styles got applied right, but other styles are wonky. Diffing between the two, we see that no direct child selectors (that is, ones like .a > .b) are working. And looking in the rendered source, we find the culprit: they’re being written as .a > .b. Huh??
innerHTML
Some background on innerHTML: it has some simple character escapes built-in, from some sort of “fragment parsing algorithm” that detects if something should be text or should be an HTML element. This makes code like this “just work”:
const test = (doc, elemType, text) => {
const elem = doc.createElement(elemType);
elem.innerHTML = text;
console.log(elem.innerHTML);
};
test(document, "p", "2 + 2 < 5");
// prints "2 + 2 < 5"
Now, because CSS wants unescaped symbols, the usual escaping rules seemingly do not apply to it:
test(document, "style", "p > h2 { color: blue; }");
// prints "p > h2 { color: blue; }"
…except, of course, when you have a virtual Document
test(new Document(), "style", "p > h2 { color: blue; }");
// prints "p > h2 { color: blue; }"
Because of course! Why would that work? Clearly I am extremely silly for expecting otherwise.
I spent a lot of time spinning my wheels on this one, probably because I had skipped lunch 2 hours ago by the time I got to this point. Having a lunch and coming back, the solution was easy: just do
const output = `<html><head><style>${rules.join("\n")}</style></head>${document.body.outerHTML}</html>`;
I didn’t need to keep everything as elements in the first place! All I cared about was the output text. A bit more magic to remove all scripts and make all images load from data URLs2, and bam it was working flawlessly. In the end I still had a lot of fun with this, hope you learned something too!