If you are seeing this message, the HTML is being displayed inside a
preview pane (email client, chat app, SharePoint, Confluence, …) that
blocks embedded JavaScript for security. The file itself is fine.
Download / save the attachment to your Desktop (do not use the in-app preview).
Right-click the saved file → Open With → Chrome, Firefox, or Edge.
On Mac: Chrome and Firefox work out of the box. Safari with file:// may also need Develop → Disable Local File Restrictions.
Once opened in a real browser the viewer is fully self-contained — no internet and no companion files required.