Should I teach modern web development in an introductory programming course? How?
If you want to teach students programming concepts along with web-development (HTML/CSS/JS) you should consider lacing programming examples or lessons with examples of HTML/CSS/JS or similar concepts in both. You could also have HTML/CSS/JS as an extra credit assignment or something the class can do if they're ahead of schedule. This will keep the class motivated to learn HTML/CSS/JS, which you probably shouldn't grade as it's an extra addition to the curriculum. This will teach the class the necessary topics along with them not being bored out of their mind, studying for programming AND web development.