In the brief history of web design, few issues have been as contentious as the question of whether designers should code.
It’s beyond question that to design for the web you need an appreciation of how sites work, but does that understanding need to be in-depth enough that you can write code—after all you expect developers to adhere to a brand guide, you wouldn’t necessarily expect them to create one.
WYSIWYGs are increasingly competent, and most produce code adequate for prototypes, or to pass on to a developer (if not actually production quality). At the same time, frontend code is more complex than it was even five years ago; it’s not at all uncommon to find frontend developers who specialize in a single technology, like CSS. So there’s both less need for designers to code, and an increasingly difficult challenge for those that choose to.
On the other hand, most designers who gravitate to the web do so through a natural curiosity. It’s a strange designer who hasn’t at least played around with developer tools in the browser. What’s more, writing HTML or CSS can’t accurately be described as coding; one is a markup, the other is a set of style definitions—both part of a designer’s job long before web design.