There is no set guideline for best design practices because technology and the industry standard change constantly. Design trends evolve constantly. Every year you'll find articles and blog posts on the designed trends of the following year. One year, flat design is in and all design elements and icons are design according to this style; in a couple of months, skeuomorphism is the next big thing and designers are designing according to this new trend, so when we speak of classic and timeless design in the following sections we're referring to not incorporating every new design fad, but rather finding a happy medium between keeping the design fresh and timeless.
Skeuomorphism is the design concept of making items represented resemble their real-world counterparts.