JSFiddle - React, Tailwind, and code Playground

JavaScript

var $char = $('<span>').css({ padding: 0, margin: 0 }).text('=').appendTo('body'),
    charWidth = $char.width(),
    numberOfChars = parseInt( ( $('body').width() / charWidth ).toFixed(), 10 );
    
$char.remove();

$('body').append( Array(numberOfChars).join('=') );