Handling <?xml-stylesheet> similar to <link rel="stylesheet">?

Question

Handling <?xml-stylesheet> similar to <link rel="stylesheet">?

623 views Asked by AudioBubble At 06 January 2017 at 00:55

During investigation of advantages and disadvantages of attaching CSS with <?xml-stylesheet> processing instruction, I came upon some issues.

Suppose we have a simple XHTML document (which is delivered with application/xhtml+xml MIME type and viewed in a Web browser):

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>A sample XHTML document</title>
    <script type="application/javascript" src="/script.js"></script>
  </head>
  <body>
    <h1>A heading</h1>
  </body>
</html>

Then we have an external CSS file (let it be named style.css and put in root directory):

h1 { color: red; }

At first, in script.js, I dynamically attach this CSS with a link element:

const link = document.createElement('link');
Object.entries({rel: 'stylesheet', type: 'text/css', href: '/style.css'})
      .forEach(([name, value]) => link.setAttribute(name, value));
document.head.appendChild(link);

Then the script is waiting until the stylesheet finishes loading and reaches it through sheet property:

link.addEventListener('load', function() {
  const stylesheet = link.sheet;
});

After this, the script can manipulate this stylesheet, for example:

stylesheet.cssRules.item(0).style.color = 'green';      // modify an existing rule
stylesheet.insertRule('body { background: #ffc; }', 1); // insert a new rule

But now, I cannot figure out whether the same manipulations are possible if a stylesheet is attached with <?xml-stylesheet> processing instruction:

const pi = document.createProcessingInstruction('xml-stylesheet',
           'href="/style.css" type="text/css"');
document.insertBefore(pi, document.documentElement);

First, PI seem not to have load event, so the script cannot know when the stylesheet is ready. Second, there is nothing like sheet property, so you cannot call pi.sheet to reach the stylesheet.

Is there any way to overcome these difficulties and to get from the script to the stylesheet associated with <?xml-stylesheet> PI?

Original Q&A

There are 1 answers

**guest271314** · Answer 1 · 2017-01-06T19:07:17+00:00

First, PI seem not to have load event, so the script cannot know when the stylesheet is ready.

You can use PerformanceObserver to check for requested and loaded resources. Iterates nodes of document, check for .nodeType 7 or .nodeType 8, as ProcessingInstruction node could have comment .nodeType. Get "resource" property from performance entries. Parse .nodeValue of filtered node for URL at href="URL", check if value is equal to "resource" of performance entry, then check if .styleSheet .href value is equal to parsed URL, and if parsed URL is equal to performance entry "resource" property value. If true, iterate .cssRules or .rules of the styleSheet loaded at ProcessingInstruction node.

window.onload = () => {
  let resource;
  const observer = new PerformanceObserver((list, obj) => {
    for (let entry of list.getEntries()) {
      for (let [key, prop] of Object.entries(entry.toJSON())) {
        if (key === "name") {
          resource = prop;
          var nodes = document.childNodes;
          _nodes: for (let node of nodes) {
            if (node.nodeType === 7 || node.nodeType === 8 
            && node.nodeValue === pi.nodeValue) {
              let url = node.baseURI 
                        + node.nodeValue.match(/[^href="][a-z0-9/.]+/i)[0];
              if (url === resource) {
                observer.disconnect();
                // use `setTimeout` here for
                // low RAM, busy CPU, many processes running
                let stylesheets = node.rootNode.styleSheets;
                for (let xmlstyle of stylesheets) {
                  if (xmlstyle.href === url && url === resource) {
                    let rules = (xmlstyle["cssRules"] || xmlstyle["rules"]);
                    for (let rule of rules) {
                      // do stuff
                      console.log(rule, rule.cssText, rule.style, xmlstyle);
                      break _nodes;
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  });

  observer.observe({
    entryTypes: ["resource"]
  });

  const pi = document.createProcessingInstruction('xml-stylesheet',
    'href="style.css" type="text/css"');
  document.insertBefore(pi, document.documentElement);

}

plnkr http://plnkr.co/edit/uXfSzu0dMDCOfZbsdA7n?p=preview

You can also use MutationObserver, setTimeout() to handle

low RAM, busy CPU, many processes running

window.onload = function() {
  let observer = new MutationObserver(function(mutations) {
    console.log(mutations)
    for (let mutation of mutations) {
      for (let node of mutation.addedNodes) {
        if (node.nodeName === "xml-stylesheet") {
          let url = node.baseURI 
                    + node.nodeValue.match(/[^href="][a-z0-9/.]+/i)[0];
          setTimeout(function() {
            for (let style of document.styleSheets) {
              if (style.href === url) {
                observer.disconnect();
                // do stuff
                console.log(style)
              }
            }
          // adjust `duration` to compensate for device
          // low RAM, busy CPU, many processes running
          }, 500)  
        }
      }
    }
  });

  observer.observe(document, {
    childList: true
  });

  const pi = document.createProcessingInstruction('xml-stylesheet',
    'href="style.css" type="text/css"');
  document.insertBefore(pi, document.documentElement);

}

plnkr http://plnkr.co/edit/AI4QZiBUx6f1Kmc5qNG9?p=preview

Alternatively, use XMLHttpRequest() or fetch() to request .css file, create and append <style> element to document, do stuff with response text, set .textContent of style element to adjusted css text.

TechQA.

Handling <?xml-stylesheet> similar to <link rel="stylesheet">?

There are 1 answers

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