Improve frontend guideline (go-gitea#28711)

It has been a consensus for long time during reviewing: "avoiding inline
styles as much as possible".

docs/content/contributing/guidelines-frontend.en-us.md

@@ -48,11 +48,12 @@ We recommend [Google HTML/CSS Style Guide](https://google.github.io/styleguide/h
 10. Avoid mixing different events in one event listener, prefer to use individual event listeners for every event.
 11. Custom event names are recommended to use `ce-` prefix.
 12. Gitea's tailwind-style CSS classes use `gt-` prefix (`gt-relative`), while Gitea's own private framework-level CSS classes use `g-` prefix (`g-modal-confirm`).
+13. Avoid inline scripts & styles as much as possible, it's recommended to put JS code into JS files and use CSS classes. If inline scripts & styles are unavoidable, explain the reason why it can't be avoided.
 
 ### Accessibility / ARIA
 
 In history, Gitea heavily uses Fomantic UI which is not an accessibility-friendly framework.
-Gitea uses some patches to make Fomantic UI more accessible (see the `aria.js` and `aria.md`),
+Gitea uses some patches to make Fomantic UI more accessible (see `aria.md` and related JS files),
 but there are still many problems which need a lot of work and time to fix.
 
 ### Framework Usage