Most developers can code HTML + CSS in a pinch, but doing it really well requires more forethought and skill than you can expect from a jack-of-all-trades developer.
I specialize in markup and style. I’m comfortable not only with vanilla HTML5 but with related templating languages such as Twig, Liquid, HAML, Handlebars and Jade. I can write templates for any sort of app or CMS but I’m particularly fond of Grav, Ghost, and Shopify.
I’m an expert in CSS, well versed in modern goodies like keyframe animations and flexbox, and obscure layout gotchas like static vs relative positioning, the way the Z axis works when nesting objects, the situational advantages of floats vs inline blocks, border-box vs content-box, etc.
I prefer to write my style in SCSS, with a concise and organized structure inspired by SMACCS and Atomic Design. I’m familiar with Bootstrap, Foundation and the rest but prefer the more elegant and modern Bourbon and Neat, adding in my own Sass variables, functions and mixins when appropriate.
I’m also comfortable with most of the things that come with web development: Git & Github, DNS settings, homebrew, FTP / SFTP / SSH, cloud servers and deployment.
Again, I’m a specialist. I don’t consider myself a programmer per se, so if you’re creating a unique app, I shouldn’t be your only developer.