JAVASCRIPT

add style javascript

// Bad:
element.setAttribute("style", "background-color: red;");
// Good:
element.style.backgroundColor = "red";

Add style to the HTML document with JS

document.head.insertAdjacentHTML("beforeend", `<style>body{background:red}</style>`)

style through javascript

element.style.backgroundColor = "red";   // set the background color of an element to red