tabs.insertCSS(): Improve text

### MDN URL

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS

### What specific section or headline is this issue about?

cssOrigin

### What information was incorrect, unhelpful, or incomplete?

> "user" enables you to prevent websites from overriding the CSS you insert: see [Cascading order](https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Cascade/Introduction#cascading_order).

The statement can lead to confusion as "user" will only overrides 'author' (websites) if `!important` is used.

Similarly:
- [scripting/insertCSS#origin](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/insertCSS#origin)

---
Unify statement

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/insertCSS#origin
This property is case-insensitive from Firefox 144.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/RegisteredContentScript#cssorigin
This property is case insensitive in Firefox and Safari.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contentScripts/register#cssorigin
This property is case insensitive.

### What did you expect to see?

As above

### Do you have any supporting links, references, or citations?

_No response_

### Do you have anything more you want to share?

### MDN metadata

Page report details

* Folder: `en-us/mozilla/add-ons/webextensions/api/tabs/insertcss`
* MDN URL: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS
* GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/mozilla/add-ons/webextensions/api/tabs/insertcss/index.md
* Last commit: https://github.com/mdn/content/commit/85fccefc8066bd49af4ddafc12c77f35265c7e2d
* Document last modified: 2025-11-07T15:58:06.000Z