Rendering HTML inside textarea
This is not possible to do with a textarea. What you are looking for is an content editable div, which is very easily done:
<div contenteditable="true"></div>div.editable { width: 300px; height: 200px; border: 1px solid #ccc; padding: 5px;}strong { font-weight: bold;}<div contenteditable="true">This is the first line.<br>See, how the text fits here, also if<br>there is a <strong>linebreak</strong> at the end?<br>It works nicely.<br><br><span style="color: lightgreen">Great</span>.</div>
With an editable div you can use the method document.execCommand (more details) to easily provide the support for the tags you specified and for some other functionality..
#text { width : 500px; min-height : 100px; border : 2px solid;}<div id="text" contenteditable="true"></div><button onclick="document.execCommand('bold');">toggle bold</button><button onclick="document.execCommand('italic');">toggle italic</button><button onclick="document.execCommand('underline');">toggle underline</button>
Since you only said render, yes you can. You could do something along the lines of this:
function render(){ var inp = document.getElementById("box"); var data = `<svg xmlns="http://www.w3.org/2000/svg" width="${inp.offsetWidth}" height="${inp.offsetHeight}"><foreignObject width="100%" height="100%"><div xmlns="http://www.w3.org/1999/xhtml" style="font-family:monospace;font-style: normal; font-variant: normal; font-size:13.3px;padding:2px;;">${inp.value} <i style="color:red">cant touch this</i></div></foreignObject></svg>`; var blob = new Blob( [data], {type:'image/svg+xml'} ); var url=URL.createObjectURL(blob); inp.style.backgroundImage="url("+URL.createObjectURL(blob)+")"; } onload=function(){ render(); ro = new ResizeObserver(render); ro.observe(document.getElementById("box")); }#box{ color:transparent; caret-color: black; font-style: normal;/*must be same as in the svg for caret to align*/ font-variant: normal; font-size:13.3px; padding:2px; font-family:monospace;}<textarea id="box" oninput="render()">you can edit me!</textarea>textarea will render html!Besides the flashing when resizing, inability to directly use classes and having to make sure that the div in the svg has the same format as the textarea for the caret to align correctly, it's works!