I'd love to only have to specify a single SVG image for a site and have it used for all sizes and scales.

SVG support feels like it should be pretty straightforward to implement.

Last time I looked at how to implement favicons “correctly” I had to make some weird XML file with the word apple in it and it lists all the size of icons I have, and then use a favicon generator to create all the “correct” sizes.

What a mess.

but note that browsers rate-limit you to approx. 1 update per sec

    data:text/html;charset=utf-8,<link rel="icon" href="
    data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'>
    <g>
    <text x='0,0,100,0,200,0,300,0,400,0,500,0,600,0,700,0,800,0,900,0,1000,0,1100,0,1200,0,
             1300,0,1400,0,1500,0,1600,0,1700,0,1800,0,1900,0,2000,0,2100,0,2200,0,2300'
          y='.9em' font-size='90'></text>
    <animateTransform attributeName='transform'
                      values='0,0;-200,0;-400,0;-600,0;-800,0;-1000,0;
                              -1200,0;-1400,0;-1600,0;-1800,0;-2000,0;-2200,0'
                      calcMode='discrete' dur='1s' repeatCount='indefinite'/>
    </g>
    </svg>
    ">

Used it as a nice intro project to try out cloudflare hosting.