Design and art have always required technical knowledge of tools and the medium, and knowing HTML and CSS is no different.
Web application design (which is distinct from brochure web design or print design) has unique conventions and constraints, and web application designers absolutely must have a deep understanding of both. There are header nav bars. The branding with the link to the root path is in the upper left, user info/settings are in the upper right. HTML and CSS are structured as boxes within boxes. Main content areas contain the primary content, sidebars contain secondary content. Components are modular. Browsers have significant variation in how they render things, and viewport sizes vary dramatically. We have views and forms, GETs and POSTs. Resources are nested, with one URL per resource. Browsers have different levels of CSS support, leading to a focus on progressive enhancement and graceful degradation.
Every single time I've ever worked on a web application with a designer who didn't know HTML/CSS or a web designer who only does brochure sites, they stumble on these issues, and we end up with inflexible designs that have usability issues and are a pain or impossible to implement. Designers who don't know HTML/CSS create drag in a startup environment. It forces others to spend time and energy explaining how the web works and what the conventions are.
Web application design (which is distinct from brochure web design or print design) has unique conventions and constraints, and web application designers absolutely must have a deep understanding of both. There are header nav bars. The branding with the link to the root path is in the upper left, user info/settings are in the upper right. HTML and CSS are structured as boxes within boxes. Main content areas contain the primary content, sidebars contain secondary content. Components are modular. Browsers have significant variation in how they render things, and viewport sizes vary dramatically. We have views and forms, GETs and POSTs. Resources are nested, with one URL per resource. Browsers have different levels of CSS support, leading to a focus on progressive enhancement and graceful degradation.
Every single time I've ever worked on a web application with a designer who didn't know HTML/CSS or a web designer who only does brochure sites, they stumble on these issues, and we end up with inflexible designs that have usability issues and are a pain or impossible to implement. Designers who don't know HTML/CSS create drag in a startup environment. It forces others to spend time and energy explaining how the web works and what the conventions are.