src/tools/error_index_generator/error-index.css

code.compile_fail {
	border-left: 2px solid red;
}

pre .tooltip {
	position: absolute;
	left: -25px;
	top: 0;
	z-index: 1;
	color: red;
	cursor: pointer;
}
pre .tooltip::after {
	display: none;
	content: "This example deliberately fails to compile";
	background-color: #000;
	color: #fff;
	border-color: #000;
	text-align: center;
	padding: 5px 3px 3px 3px;
	border-radius: 6px;
	margin-left: 5px;
}
pre .tooltip::before {
	display: none;
	border-color: transparent black transparent transparent;
	content: " ";
	position: absolute;
	top: 50%;
	left: 2px;
	margin-top: -5px;
	border-width: 5px;
	border-style: solid;
	height: 0px;
}

pre .tooltip:hover::before, pre .tooltip:hover::after {
	display: inline;
}