Markup inside lets you add arbitrary HTML content inside the element tag. This replaces the tag content.
This can be used to embed things like a YouTube video or a Twitter post.
Use the more details button on the right side of the field to show a larger code entry field.
If you want your custom HTML to inherit a specific set of text styles add a text element to the artboard and then set the markup inside. Or you can define text styles in the page or style template.
Note: You may need to set the width and height of the nested HTML to 100% to match the size you are seeing in the artboard.
Note: Since you can include any HTML you can potentially break your element or page.