The two most important codes or markup languages that anchor all visual design in web browsers and mobile phones are HTML and CSS. I am fluent in both.
HTML defines the layout of templates in a browser and on mobile devices. Websites must be programmed responsively so that their layout adapts well to mobile. I understand exactly how this functions from the code side which makes me a far more effective visual designer on the front end.
CSS is how the elements of a layout are sized, type styles, colors, and responsive resizing that must occur between desktop and mobile. I am able to author CSS ground up and also make tweaks in existing code. This helps to move things along at a faster pace and with greater accuracy.