TL:DR;
Quick answer would be that you can't add text to a custom icon library html tag. Here is how it should be formatted.
Based on the documentation from Material Icons, you would need to have something like this :
<a rel="nofollow" class="comment-reply-link">
<span class="material-icons">reply</span> Reply
</a>
Documentation : http://google.github.io/material-design-icons/#using-the-icons-in-html
Long answer
You can use custom icon library like Material Icons
or Font Awesome
.
Let's assume you want to use Font Awesome.
Go to https://fontawesome.com/start to start creating a self-hosted icon kit. You might need to create a free account before creating the kit.
![Start a New Project with a Kit](https://i.stack.imgur.com/rp4kE.png)
Just give the name of your project and click on Create & Use This Kit.
Once you have create your kit, you will have a kit link that looks like this :
<script src="https://kit.fontawesome.com/XXXXXXXXX.js" crossorigin="anonymous"></script>
Copy this code and head over your header.php
file in your WordPress project.
Once you are in, go in the <head>
section and paste your code into it.
<!doctype html>
<html <?php language_attributes(); ?>>
<head>
<meta charset="<?php bloginfo( 'charset' ); ?>" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<link rel="profile" href="https://gmpg.org/xfn/11" />
<?php wp_head(); ?>
<script src="https://kit.fontawesome.com/XXXXXXXXX.js" crossorigin="anonymous"></script>
</head>
<body>
...
Note : It is recommended to use wp_enqueue_script()
function. You can find the documentation on the WordPress Codex : https://developer.wordpress.org/reference/functions/wp_enqueue_script/
You can save your file and now go into your template file where you want to edit your HTML.
You can browse the Font Awesome Free icons and find an icon that fits your need : https://fontawesome.com/icons?d=gallery&m=free
Once you have your icon, simply copy-paste the html tag into your WordPress template file.
![fas fa-reply](https://i.stack.imgur.com/1Nmjo.png)
<a rel="nofollow" class="comment-reply-link">
<i class="fas fa-reply fa-flip-horizontal"></i> Reply
</a>
When reloading your page, you will be able to see the icon.