you have chosen to open ““which is a: hypertext error

This thread illustrates a common operating-system file-association issue rather than a broken HTML file. @MidiMagic's use of "syntax" can be read two ways: in coding it means structure of markup, but in this context it more likely referred to the command the OS uses to launch a file (which app is selected). @ritarazzle's later observation shows the symptom was resolved by changing which application the OS used to open that file type.

A short troubleshooting checklist that covers the typical causes and fixes:

• Confirm the file has the correct extension (for example, .html or .htm) and that extensions are not hidden or accidentally changed.
• Reset the system default for that extension: on macOS use Get Info (Command-I) -> "Open with" -> pick the browser -> "Change All"; on Windows use Right-click -> Open with -> Choose default program and check "Always use this app".
• In Dreamweaver, verify Preview/Browser preferences so the desired browsers appear in the preview list and the correct one is selected when using "Preview in Browser".
• If a page still shows errors in a browser, open it directly from the browser (File > Open) or serve it via a local webserver (http://localhost) to rule out differences between file:// and http:// access.

Cautions: avoid changing extensions by simple renaming without checking the true file type, and when uploading to a server ensure the file is saved with the right extension so the server sends the correct content type. This approach addresses the same class of problem discussed by @itisnot_me and the other replies.