<!doctype html><style>
:root {
--huge: 5px;
}
svg {
width: 100px;
height: 100px;
border: 1px solid black;
}
</style><svg viewBox="0 0 10 10"><rect x=4.5 y=4.5 width=1 height=1 stroke="green" stroke-width="var(--huge)" />