How to save a HTML document 

1. Go to File > Save As.
2. File Name: Give it a name and manually add the .html extension (e.g., index.html or myfirstpage.html).
3. Save as type: Crucially, change this dropdown to “All Files” (or similar) to prevent the computer from automatically adding a .txt extension.
4. Encoding: Select UTF-8, which is the recommended encoding for HTML5.
5. Click Save.