CSS/HTML themes/templates:
Free CSS*,
HTML5 UP!,
Blaze UI, modern-normalize, Normalize.css, Reseter.css, Shoelace
Blocks:
blocks.css,
paper.css,
wickedblocks*
Minimalism:
chota (GitHub),
Water.css
Classless:
classless-css list💩,
Almond.css,
CSS Bed,
Simple.css (GitHub)
Bootstrap 4 Cheat Sheet*, codrops' CSS Reference*
Blobs:
MagicPattern's Blob Generator☁
Triangles:
10015's CSS Triangle Generator☁
Clip path:
10015's CSS Clip Path Generator☁
Cubic bezier:
10015's Cubic Bezier Generator☁
Centering:
How to Center in CSS*
Loader element:
10015's CSS Loader Generator☁
Responsive grids:
ungrid (GitHub)
Responsive grid generator:
CSS Grid Generator☁ (GitHub),
Interactive CSS Grid Generator☁ (GitHub)
Color filters:
filters.css (GitHub)
Dark mode:
Night Eye's Dark CSS Generator☁
Gradients:
10015's CSS Gradient Generator☁
CSS Gradient*
Gradients with hover effects:
Colorion's Gradient Buttons*
Image hover effects:
imagehover.css☁ (GitHub)
Keyframes:
Keyframes.app's Animate☁
SVG animations:
Vivus Instant☁ (GitHub)
Border radius:
10015's CSS Border Radius Generator☁
Frosted glass:
10015's CSS Glassmorphism Generator☁,
Glassmorphism☁
Shadows:
10015's CSS Box Shadow Generator☁,
brumm.af's SmoothShadows☁,
Keyframes.app's Animate☁,
Trendy CSS Text Shadows by Mixfont☁
Geometric patterns:
10015's CSS Background Pattern Generator☁,
CSS3 Patterns Gallery*,
Hero Patterns*,
MagicPattern's CSS Background Patterns☁
patterns.css* (GitHub),
Subtle Patterns,
Transparent Textures
Live backgrounds:
makebackground.io☁
Triangle patterns:
Trianglify.io☁ (with 🔌Trianglify JS library)
Utility-first:
Tailwind CSS
Data visualizations:
Charts.css
Automatic theme generation:
Bootstrap Magic☁ (GitHub)
CSS reset:
a11y-css-reset,
Bootstrap themes/templates:
Bootswatch,
Creative Tim's Free Templates,
Material Design for Bootstrap 5 & 4
Tailwind components:
daisyUI,
Tailwind UI
Tailwind UI Kit
A11y issues:
a11y.css⇉ (GitHub),
Checka11y.css (GitHub)
Validation/linting:
CSS Lint☁
Minifier/optimizing:
10015's CSS Minifier☁