JSFiddle - React, Tailwind, and code Playground

HTML

<p>CTRL-B and CTRL-I should work: 
<div contenteditable class="editor">
      Sample template with <input type=button class="mergecode" value="MergeCode1">.
</div>

CSS

.editor { font: 9.5pt serif; height:200px; border:1px dashed red; margin:10px; }

.mergecode { 
    font-family:inherit;
    font-size:inherit;
    font-weight:inherit;
    background:gold; border:1px solid black;
    padding: 2px 4px; }