Understanding how different colours are perceived by different people is an important responsibility for designers. Although the use of contrast checkers is becoming commonplace, it's rare for designers to question how contrast algorithms work and why contrast changes across different hues.

I've found learning about the science of colour to be a fascinating subject (Nate Baldwins colorandcontrast.com is a great place to start) and a subject which has greatly improved my work, especially when designing for multiple brands.

When auditing products and sites, the relationship between semantics and style is one of the areas that almost always causes issues.

Typography scales are traditionally expressed using semantic HTML structures like headings and paragraphs. Semantics help assistive technology users navigate digital content, however the naming conventions are often seen by designers as simply a way to specify styles. Heading structures quickly break down when designers are tempted to use different font sizes in an order that makes little sense from a semantic perspective.

That why I usually recommend uncoupling typography styles from HTML tags. Although I would always try to align visual and semantic structures, avoiding the casual use of a h4 instead of a h2 is often the best option.

Giffgaff design have a great example of this approach, and the Gusto Design Team have written about their experiences choosing naming conventions for typography tokens.