CSS: "This property is invalid"

CSS I want in BSS:

.btn-toggle::before {
    width: 1.25em;
    line-height: 0;
    content: url(data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' width='16' height='16' viewBox='0 0 16 16'%3e%3cpath fill='none' stroke='rgba%280,0,0,.5%29' stroke-linecap='round' stroke-linejoin='round' stroke-width='2' d='M5 14l6-6-6-6'/%3e%3c/svg%3e);
    transition: transform .35s ease;
    transform-origin: 0.5em 50%;

BSS says “This property is invalid”:

PS: Do I have to add ::before to a button or is it done automatically?

Try putting the data in double quotes

content: url("your data")

My also setting to not, BSS sometimes gives errors in css that work in the browser anyway