A webpage is not always broken because of a large mistake. Small details can produce exactly this problem: debugging: custom element does not upgrade. This guide shows you what to look for, a concrete fix, and how to check that it worked.
The Idea in Plain English
Custom element names need a hyphen and a matching registration call. A JavaScript load error can prevent upgrade.
You can test this using a small local HTML file. Change one part at a time and check the result in the browser before adding more code.
A Working Example
The first part shows a problem. The second part shows a safer or more accurate version. Compare them before reading the explanation.
<!-- Problem -->
<my-card>Ready</my-card>
<!-- Corrected example -->
<my-card>Ready</my-card>
<script>
class MyCard extends HTMLElement {}
customElements.define("my-card", MyCard);
</script>What Each Part Does
- The script element can load code or hold data; the type attribute tells the browser how to treat its contents.
- The browser reads these lines in order and uses them to build the page or request.
- Changing the relevant value lets you test what this feature controls.
Before the fix, the browser can behave incorrectly or report a warning. After the change, test the exact failure rather than assuming the page is fixed.
Try It Yourself
Check customElements.get("my-card") in the console.
Common Beginner Mistakes
- Check the underlying cause before changing unrelated tags.
- Changing several parts of the example at once, making it difficult to see which part caused a different result.
- Choosing a tag only because its default appearance looks right, without checking whether it describes the content correctly.
Key Takeaways
- Custom element names need a hyphen and a matching registration call. A JavaScript load error can prevent upgrade.
- Before the fix, the browser can behave incorrectly or report a warning. After the change, test the exact failure rather than assuming the page is fixed.
- Test the markup in a browser, and check accessibility and real-world behavior where relevant.