I've only been doing government-related web dev for a decade, but what about WCAG 2? Are accessibility guidelines not a design standard? They seem to cover issues like contrast, at least.
It’s good they exist, but accessibility is only one aspect of design. The HIG tell you how far a button should be from the edge of the screen. They tell you what specific colors to use for certain items.
Also I’ll just point out that a standard is only meaningful if it’s widely used. We could debate how widely used WCAG 2 is, but it’s not anywhere near other platform standards.
https://www.w3.org/TR/WCAG21/
Is it really that hard to run your website through WAVE and fix the issues it identifies?
https://wave.webaim.org/
You don't even need the WAVE website; they provide add-ons for Firefox and Chrome.