Why is it an antipattern?
Designing a HTML page can be complicated, especially if client requires complicated things, animations and other effects, pixel precision, and compatibility with older browsers. Also the page may contain non-trivial JavaScript code. Mix it up with PHP code (or Java code), and you get something very complicated. If you do some change in design, it may break the code. If you do some change in code, it may break the design. In a larger team you can have one person who is HTML expert but not a programmer (or a very bad programmer), and a coder who is not a HTM...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.