You can encode a Data URI of an SVG that contains a text element (source):
<link rel="icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>??</text></svg>">
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…