You don't. HTML is not an image file format. HTML is a vanilla ASCII text
file that may include instructions to the web browser processing it where it
is to go to retrieve and display an image file actually stored as a GIF,
JPG, PNG, or BMP file. In MS Project you can use the little camera on the
tool bar to save the Gantt chart view as a GIF image file which can then be
referenced using an image tag in an HTML source file.