What does <number>%<number> mean in a CSS length, eg 100%25

I came across some code today where the author declared a length as <numeric value>%<numeric value>

CSS: width: 100%25;
HTML/SVG: width="100%25"

I have been writing CSS for years and I have never seen a length declared like this. It seems to work in HTML, SVG and CSS.

Is it some form of shorthand? What does it mean? I have tried searching for answers, but the nature of the topic made it hard to find something.

Original code:

<svg viewBox='0 0 900 900' xmlns='http://www.w3.org/2000/svg'>
    <foreignObject width='100%25' height='100%25'>
        <div xmlns='http://www.w3.org/1999/xhtml'>
            <style>
                .c{
                    display:flex;
                    align-items:center;
                    justify-content:center;
                    margin:0;
                    width:100%25;
                    height:900px
                }
                .b{
                    width:33.33%25;
                    height:33.33%25;
                    background:red
                }
            </style>
        <div class='c'>
            <div class='b'/>
                
            </div>
        </div>
    </foreignObject>
</svg>