JSFiddle - React, Tailwind, and code Playground
HTML
<textarea><font color='#fbffff'>abc<font size='1'>zx</font>dc</font></textarea>
JavaScript
var txt = $('textarea').val();
txt = txt.replace(/\<font.*?color\s*=\s*["']\s*#fbffff\s*["'].*?\>(.*?)(\<font.*?\>.*?\<\/font\>.*?)\<\/font\>/gim, '$1$2');
$('textarea').val(txt);