How to get list of Css class use in the HTML file?

Pure Javascript Code will list all the unique classes.

var lst=[];
document.querySelectorAll("[class]").forEach( (e)=>{  
e.getAttribute("class").split(" ").forEach( (cls)=>{if( cls.length>0 && lst.indexOf(cls)<0) lst.push(cls);}
);
 });
console.log(lst.sort());

This should work and it doesn't need jquery:

const used = new Set();
const elements = document.getElementsByTagName('*');
for (let { className = '' } of elements) {
    for (let name of className.split(' ')) {
        if (name) {
            used.add(name);
        }
    }
}
console.log(used.values());

If you've got jQuery on the page, run this code:

var classArray = [];
$('*').each(function(){if(this.className!=""){classArray.push(this.className)}})

The variable classArray will contain all the classes specified on that HTML page.