I agree that the question is slightly incorrect. Difficult can be the process of layout for a single coder, and the coding should be correct.
First, the code must be valid, well structured and commented. For all of the developers who will then do something with the layout (e.g. the backend), will thank you.
In General, here's a few pointers.
Tatum_White21 answered on July 9th 19 at 11:27
For each different!
Someone animations, animated effects!
Someone support for IE8 and other older versions of browsers, and someone, on the contrary, difficult given the new html5 features (flexbox as an example), adaptability!
Someone many pages with many elements, because this is long and tedious.
And someone where a lot of js.
And to the generally accepted concept of "complex typesetting", something I've never heard of!