Wang's D-coloring conjecture for planar graphs

Let $G$ be a planar graph with maximum degree $\Delta\ge4$.

Wang's conjecture. The D-coloring number of $G$ satisfies

$$\chi'_D(G)\le \begin{cases} 9, & \Delta=4,\\ 10, & \Delta=5,\\ 2\Delta-1, & \Delta\ge6. \end{cases}$$

The bound is motivated by planar book constructions and by known upper bounds for the related B-coloring problem; the conjecture remains open only for $6\le\Delta\le32$.