Missing Colon (:) in position property: In the .btn::after rule:
Invalid Nested Selector Syntax: SCSS-like nesting is used, but plain CSS doesn't support nesting.
Incorrect Selectors for Pseudo-elements: You have &::before, &::after, and & .fa, which is valid SCSS, but invalid in CSS. These need to be un-nested.
Usage of svg within a class: In .logo svg: It references an svg element directly inside a .logo class. This should work fine if there is an svg within the .logo element, but make sure the structure matches.
Vendor Prefix Missing for appearance: The appearance property inside .select may require vendor prefixes for better browser support:
Potential Confusion with Deep Selectors: The use of :-moz-focusring selector inside .select could limit the support to Firefox only, making it inconsistent across browsers.
Shadow/Opacity on Images: The background image (background: url('http://cdn.flaticon.com/svg/44/44386.svg')) under .credit-card-box .front::before has an opacity of 0.05. This could make the image too faint. You might want to increase it.
box-shadow Issue: For .credit-card-box .back, the heavy use of box-shadow might result in performance degradation if used on lower-end devices, especially since the element already has other transformations.
Missing Colon (:) in position property: In the .btn::after rule: Invalid Nested Selector Syntax: SCSS-like nesting is used, but plain CSS doesn't support nesting. Incorrect Selectors for Pseudo-elements: You have &::before, &::after, and & .fa, which is valid SCSS, but invalid in CSS. These need to be un-nested. Usage of svg within a class: In .logo svg: It references an svg element directly inside a .logo class. This should work fine if there is an svg within the .logo element, but make sure the structure matches. Vendor Prefix Missing for appearance: The appearance property inside .select may require vendor prefixes for better browser support: Potential Confusion with Deep Selectors: The use of :-moz-focusring selector inside .select could limit the support to Firefox only, making it inconsistent across browsers. Shadow/Opacity on Images: The background image (background: url('http://cdn.flaticon.com/svg/44/44386.svg')) under .credit-card-box .front::before has an opacity of 0.05. This could make the image too faint. You might want to increase it. box-shadow Issue: For .credit-card-box .back, the heavy use of box-shadow might result in performance degradation if used on lower-end devices, especially since the element already has other transformations.