Get HTMLElement from Element

You just need to cast it:

let nodes = document.querySelectorAll('a');
for (let i = 0; nodes[i]; i++) {
    let node = nodes[i];
    var c = (nodes[i] as HTMLElement).style.backgroundColor = 'red';
}

You can even cast to a more specific element:

(nodes[i] as HTMLAnchorElement).style.backgroundColor = 'red';

The thing is that document.querySelectorAll returns the most generic element type, but if you know yourself what is the specific type then you can cast, because you "know better" than the compiler.

let nodes = document.querySelectorAll<HTMLElement>('a');
// or
let nodes = document.querySelectorAll<HTMLAnchorElement>('a');