When creating a design for an interactive video designers and developers should take 'responsive design' into account.
Depending on how the video will be served to viewers it can be displayed in many different sizes. It is important that all your interactive elements keep readability and functionality in all circumstances. Therefore a dynamic font-size and relative sizing/positioning can be useful.
A detailed technical view on this can be read here.
Sometimes, interactive elements will become too small when the video is viewed on mobile devices or in small embed sizes. You can avoid this by creating a separate design that is automatically triggered on smaller sizes. Today most websites do this to make sure everyone can access its content easily.