Between CSS and JavaScript is a weird symmetry. CSS style rules are laid on top of the DOM. The CSS property names like "font-weight" are transliterated into "myElement.style.fontWeight". The class of an element can be swapped out. For example:

document.getElementById("myText").style.color = "green";

document.getElementById("myText").style.fontSize = "20";

-or-

document.getElementById("myText").className = "regular";