When a browser loads an HTML page, its convert it to the Document Object Model (DOM).
The browser's produced HTML DOM, constructs as a tree that consists of all your HTML page element as objects. For example, assuming that you load below HTML page on a browser:
<!DOCTYPE html><html><head><title>website title</title></head><body><p id="js_paragraphId">I'm a paragraph</p><a href="http://somewebsite.com">some website</a></body></html>After loading, the browser converts it to:
Some of the ability of scripting languages on HTML DOM consists of:
1- Change all the HTML elements in the page.
2- change all the HTML attributes in the page.
3- Change all the CSS styles on the page.
4- Remove existing HTML elements and attributes.
5- Add new HTML elements and attributes.
6- React to all existing HTML events in the page.
7- create new HTML events on the page.
Let's back to your questions:
1- It currently used in all modern browsers.
2- Front-end developers.
3- All Front-end developers that using scripting languages especially JavaScript.