You can create a new HTML document based on any existing HTML file. If you set up a path to a directory containing HTML templates in Manage Files, these template choices appear in a drop-down list when you create a new document. All links to assets such as images, CSS, and JS files are maintained when you create a new HTML document from a template in Content.
Therefore, you can build many documents from one central template housed in the template directory, with standard styles and images already included.
If you have already created an entirely new HTML document, you can save it in your template directory for other documents.