The minimal angle condition for quadrilateral finite elements of arbitrary degree

We study $W^{1,p}$ Lagrange interpolation error estimates for general quadrilateral ${\mathcal{Q}}_k$ finite elements with $k\ge 2$. For the most standard case of $p=2$ it turns out that the constant $C$ involved in the error estimate can be bounded in terms of the minimal interior angle of the quadrilateral. Moreover, the same holds for any $p$ in the range $1\le p<3$. On the other hand, for $3\le p$ we show that $C$ also depends on the maximal interior angle. We provide some counterexamples showing that our results are sharp.