Do not transition them all

Transitioning all css properties might be overkill.
My two cents are to be more specific on which properties we actually want to transition.

What are vendor prefixes

Vendor prefixs are magic words for cross-browser support.
My two cents are to use online autoprefixer or some other autoprefixer tool for vendor prefixes.

Fluid CSS

CSS layout can be fluid, allowing any content to fit into container while keeping the styles and not breaking the layout.
My two cents are to use paddings instead of fixed values in height and width.

How to import google fonts

Some of my students often forget to import font-weight and font-style with font-family from the Google fonts.
My two cents are to import font styles and font-weights you need, not just the font-family.

How deep is your selector

Too specific selectors might be hard to overwrite and reuse.
My two cents are to use class instead of nested selectors.

IDs as selectors

IDs are hard to overwrite and reuse on the same page.
My two cents are not to use classes instead of IDs.

Magic numbers

Smelly css often has random number because they just work.
My two cents are to try to find logic behind the unit values.

Order of media queries matters

Order of code in CSS matters. Media queries are not an exception.
My two cents are to be careful how are we going to order media queries.

Too general selectors

Global selectors might have side effect we are not aware of.
My two cents are to use class instead of global selectors.