When you export your text as an HTML5 file, all information regarding fonts, styling and the text itself is stored within the Index.html file that is exported with your project. Ad Servers will use this file when serving your advertisement online.
On export, Flexitive leverages font subsetting. This means that only the components of the fonts that you use in your projects will be exported from Flexitive. While this is useful for reducing file size and load times of your project, you will have to remember to consider it if you are making any changes to the text in your project after exporting.