demo

vanilla-editor.css (436B)

#content_source, #content_view {
	width: 100%;
	/*
	 * I so wish height: max-content; would work for textareas…
	 * Should you need to do so, you need to put an hidden <pre> and synchronise it's content with the textarea, just like for the rich-text area.
	 */
	height: 25ch; /* fallback */
	height: 33vh;
}
/* Otherwise, elements further down ends up overlaying #content_view on small viewports */
#content_view { display: table; }