Lokalise supports plain HTML documents. These documents do not require any special formatting, but please note that they can only be uploaded to projects with Documents type. The project type is selected upon creation:

To learn more about translating HTML content, please check out our blog post.

You can also use this unofficial extension for Chrome to preview HTML documents in Lokalise before exporting them. To use the extension, simply install it, proceed to Lokalise, open the Download page, adjust options as needed, click Preview and scroll to the bottom of the page.