  We provide a geometric model for the classifying space of automorphism groups
of Hermitian vector bundles over a ring with involution $R$ such that
$\frac{1}{2} \in R$; this generalizes a result of Schlichting-Tripathi
\cite{SchTri}. We then prove a periodicity theorem for Hermitian $K$-theory and
use it to construct an $E_\infty$ motivic ring spectrum
$\mathbf{KR}^{\mathrm{alg}}$ representing homotopy Hermitian $K$-theory. From
these results, we show that $\mathbf{KR}^{\mathrm{alg}}$ is stable under base
change, and cdh descent for homotopy Hermitian $K$-theory of rings with
involution is a formal consequence.
