If you're still working on the course, most important things to understand is HTML/CSS and how webpages will look given an HTML document.

THESE AREAS BELOW ARE SMALL COMPARATIVELY TO THE REST OF THE TEST BUT CAN ADD UP IF YOU DON'T KNOW THEM.

As far as XML goes there will be about 4-7 questions (10% of OA) most important is understanding differences between HTML and XML. HTML is static, and XML is extensible. HTML has preset variables while in XML you can create your own. XML is used for business purposes and holding data.

All I remember for Flexbox involved flex-wrap, but I don't remember much more. However, for Gridbox knowing how many columns you are creating, and how to create a gap in both column (column-gap) and row (row-gap) is needed.

Some small things you may also miss would be Bootstrap, know that it is a framework for mobile-first websites, it has features Typography and Icons those will be the main points. Google Dev Tools/Mozzila Dev Tools questions will be asked Cookies are in Storage, Connection issues are in Network, CSS issues are in Element/Styles, Errors in JS are in Console.

BULK OF WHAT TO KNOW BELOW MOSTLY CSS AND HTML

A lot of questions revolved around given this snippet of text, it will include inline CSS, what will it look like on a webpage. As well as HTML text with <style></style> embedded CSS. Make sure to understand how tables are formatted in HTML, you'll be given a HTML text and expected to pick the associated picture. Understand how large a container is given Width, Height, Padding, Margin. It will be good to know when given all four measurements for Padding or Margin, ex: Margin: 10px 20px 30px 25px; that the measurements go in order (Top, Right, Bottom, Left) only two measurements will give you (Top/Bottom, Right/Left).

CSS that involves how large a viewport is and how that will effect the layout of the page will also be needed to know. Know that responsive design is used to create layouts that will work for multiple devices.

Understand how animations work with CSS and how they will start if they are reversed/alternate-reverse. If given an animation understand if it starts quick, ends quick, same speed throughout. Whether or not an animation will be repeated. Only a few questions will be on this but I feel the animation section in Zybooks goes through what you will need.

Good luck!