JSFiddle - React, Tailwind, and code Playground

HTML

<div class="editor">Hello <span id="t1">my</span><br>dear <span id="t2">world</span></div>

<button id="test">TEST</button>

CSS

.editor
{
	font-size: 16px;
}
.editor:first-line
{
	font-size: 216px;
}

JavaScript

$(document).ready(function()
{
    $('#test').click(function()
    {
            var fontSize = window.getComputedStyle(
    document.querySelector('.editor'), ':first-line'
    ).getPropertyValue('font-size');
    alert(fontSize);	
    });
});