A list of useful resources.
Developers SEO Guide | https://moz.com/blog/seo-cheat-sheet
BEM Examples | https://csswizardry.com/2013/01/mindbemding-getting-your-head-round-bem-syntax/
Coding Standards Guide | http://codeguide.co/
WordPress Template Hierarchy | https://wphierarchy.com/
Design Lessons & Tutorials | https://hackdesign.org/
HTML Validator | https://validator.w3.org/
CSS3 Generator | http://css3generator.com/
Online CSS Tester | http://csslint.net/
CSS Flex Generator | http://the-echoplex.net/flexyboxes/
CSS Gradient Generator | http://www.colorzilla.com/gradient-editor/
Online SASS & LESS testing | http://www.sassmeister.com/
Online image compression | https://tinypng.com/
Browser support checklist | http://caniuse.com/
Browser specific CSS & Javascript | http://browserhacks.com/
Browser specific CSS | https://jeffclayton.wordpress.com/category/hacks/
Redline Document | http://www.ionagroup.com/2011/05/15/teaching-web-design-the-redline-document/
Page Speed Test | https://gtmetrix.com/
Elements same height | matchHeight jQuery plugin | https://github.com/liabru/jquery-match-height
JavaScript plugin CDN | https://cdnjs.com/