Complete CSS Cheat Sheet

Hello Nick! You did a great work at composing this cheat sheet. Unfortunately, it seems that some of the sources you used were seriously outdated or not completely reliable, so there are some things in this cheat sheet that are not exactly complete or correct. Here are some of them I noticed:

background-image: CSS gradients are also values of this property. Maybe it’s worth to add them?
background-attachment: maybe it’s worth to mention the new `local` keyword?
background-clip: according to the CSS specs, length, %, and no-clip are not valid options for this property
border: wouldn’t it worth to mention that is also resets background-image?
display: there is a missing hyphen in some values (inline-block, table-header-group). Also, maybe it’s worth to add other modern values – (inline-)flex, (inline-)grid, and contents?
marquee-*: does it really belong to Box Model? Also, this CSS module is abandoned and deprecated in favor of CSS animations.
rotation: it should probably be placed in Transforms section, together with other separate transform properties.
font-family: its valid keywords are either generic names like serif, sans-serif, monospace, fantasy etc., or names of specific fonts like “helvetica”
letter-spacing: missing hyphen
word-wrap: valid values are “normal” and “nowrap”, not length & %
Grid Positioning: this module is actually outdated (as well as Template Layout module). Their modern (and well-supported) replacement is CSS Grid Layout
Template Layout: same as above. Also, “box-*” properties actually belong to the old draft of the CSS Flexbox module, not Template Layout.
:nth-last-of-type: missing hyphen
::before and ::after: these pseudo-elements insert content before/after the *content* of the element, not the element itself (i.e. inside the element, as its virtual first/last child)
px unit: in CSS, is defined as absolute unit equal to 1/96 in and has (almost) nothing to do with “pixels of viewing device”

I wish your cheat sheet to become a really useful tool for CSS beginners, and hope that good modern resources like MDN and the “official definition of CSS” from W3C, which includes the list of all CSS properties currently in use (https://www.w3.org/TR/css/#properties), will help you to fix these issues!