Get your <head> in order
Inspired by Harry Roberts' work on ct.css and Vitaly Friedman's Nordic.js 2022 presentation:
How you order elements in the <head> can have an effect on the (perceived) performance of the page.
This script helps you identify which elements are out of order.
✨ New: Install the Capo Chrome extension ✨
- Install the Chrome extension
- Explore the console logs
For applications that add lots of dynamic content to the <head> on the client, it'd be more accurate to look at the server-rendered <head> instead.
You can also use capo.js programmatically to analyze HTML <head> elements in Node.js or other JavaScript environments.
npm install @rviscomi/capo.js// Analyze a head element
const head = /* your head element */;
const adapter = new BrowserAdapter(); // Or other adapter
const result = analyzeHead(head, adapter);
console.log(result.elements); // Array of head elements with weights
console.log(result.violations); // Number of ordering violations
console.log(result.warnings); // Validation warningsLearn more about building your own adapters in the custom adapters docs.
Import only what you need for smaller bundle sizes:
// Import just the core analyzer
import { analyzeHead, checkOrdering } from '@rviscomi/capo.js';
// Import just adapters
import { BrowserAdapter } from '@rviscomi/capo.js/adapters';
// Import rules API
import { ElementWeights, getWeight } from '@rviscomi/capo.js/rules';
// Import validation API
import { isValidElement, getValidationWarnings } from '@rviscomi/capo.js/validation';analyzeHead(head, adapter)- Analyzes a head element and returns detailed resultsanalyzeHeadWithOrdering(head, adapter)- Analyzes with ordering violationscheckOrdering(elements)- Checks for ordering violations in element arraygetWeightCategory(weight)- Gets the category name for a weight value
ElementWeights- Constant object mapping element types to weight valuesgetWeight(element, adapter)- Gets the weight for a specific elementgetHeadWeights(head, adapter)- Gets weights for all elements in head
Plus individual detector functions: isMeta(), isTitle(), isPreconnect(), etc.
VALID_HEAD_ELEMENTS- Array of valid head element namesisValidElement(element, adapter)- Checks if an element is valid in headhasValidationWarning(element, adapter)- Checks if element has warningsgetValidationWarnings(head, adapter)- Gets all validation warningsgetCustomValidations(element, adapter)- Gets custom validation rules
BrowserAdapter- For working with browser DOM elementsAdapterInterface- Base interface for custom adaptersvalidateAdapter(adapter)- Validates an adapter implementation
See the migration guide for detailed migration guide.
Key changes:
- All analysis functions now require an adapter parameter
- New subpath exports for granular imports
- Enhanced TypeScript support via JSDoc
See the extension docs for detailed usage instructions.
Alternatively, you can use local overrides in DevTools to manually inject the capo.js script into the document so that it runs before anything else, eg the first child of <body>. Harry Roberts also has a nifty video showing how to use this feature. This has some drawbacks as well, for example the inline script might be blocked by CSP.
Another idea would be to use something like Cloudflare workers to inject the script into the HTML stream. To work around CSP issues, you can write the worker in such a way that it parses out the correct nonce and adds it to the inline script. (Note: Not tested, but please share examples if you get it working! 😄)
The script logs two info groups to the console: the actual order of the <head>, and the optimal order. In this collapsed view, you can see at a glance whether there are any high impact elements out of order.
Each "weight" has a corresponding color, with red being the highest and blue/grey being the lowest. See src/lib/rules.js for the exact mapping.
Here are a few examples.
Expanding the actual or sorted views reveals the detailed view. This includes an itemized list of each <head> element and its weight as well as a reference to the actual or sorted <head> element.
Here you can see a drilled-down view of the end of the <head> for the NYT site, where high impact origin trial meta elements are set too late.
