JSFiddle - React, Tailwind, and code Playground

HTML

<b id="glv">click</b>

CSS

.glavnaja-click { color:red }

JavaScript

var glavnaja = document.getElementById ('glv')
 glavnaja.onmousedown = function() {
   addClass(this, 'glavnaja-click')  
 }
 
 glavnaja.onmouseup = function() {
   removeClass(this, 'glavnaja-click')  
 }
 
 
 function addClass(el, cls) {debugger
   for(var c = el.className.split(' '),i=c.length-1; i>=0; i--) {
     if (c[i] == cls) return
   }
   
   el.className += ' '+cls
 }
 
 function removeClass(el, cls) {
   for(var c = el.className.split(' '),i=c.length-1; i>=0; i--) {
     if (c[i] == cls) c.splice(i,1)
   }
   
   el.className = c.join(' ');
 }