Without question, an area where I see young designers struggle is in the fundamentals of typography. It is especially evident when reviewing their website designs.
Think about it. Visit almost any website. What do you see? Type. Lots and lots of type. If you don't have a strong command of the fundamentals of good typography, you're never going to design a good website.
There are more than aesthetics to a successful design. Design means the website needs to be functional. Headline size. Display fonts. Body copy size. Vertical rhythm. Baseline grids. Leading. These are all fundamental concepts that the modern designer needs to master. Here is one specific example.
Did you know that famed typographer Emil Ruder figured out that 50-60 characters is the optimal line length for body copy? If the lines are too long, the eyes have a hard time identifying the next one. If they are too short, the eyes are bouncing around too often, disrupting a sense of rhythm. Readability goes far beyond making sure the type is big enough.
That is just one of the hundreds of examples. If you want to get started learning more, here are a few of my favorite resources:
The Elements of Typographic Style by Robert Bringhurst was my textbook in my college typography courses. While it focuses on print design, the basic principles apply anywhere.
Butterick's Practical Typography by Matthew Butterick is a fantastic modern teaching of type fundamentals. Perhaps most interesting about Matthew is that he's actually a lawyer, and developed an interest in design later in life. He has even created a website all about how lawyers can apply good typography to their legal documents.
Finally, Typographie: A Manual of Design by the aforementioned Emil Ruder is a classic type primer. In addition to being absolutely stuffed with informative content, the book is just stunning to behold. It is itself a masterclass in design and type fundamentals.
Become a master with typography, and everything you design will become better. Especially your designs for the web.