Thank you so much for using our theme Mercantile and for your question regarding our theme. In case of contact section(AT Contact Section Widget), we have two options, one is for Contact Form 7 shortcode and another is for page selection. Please install the contact form 7 plugin and add its shortcode there. And create a page, example Contact and add the HTML contents on Text Mode on the editor. After that your HTML will work. If the HTML have different CSS as well, you can add the CSS on Appearance > Customize > Layout/Design Options > Custom CSS as well.
Let us know, if you faced any other problem.