Tutorial: https://scotch.io/tutorials/all-favicons-for-all-devices-and-sizes
Install imagemagick:
brew install imagemagick
Creating the .ico file with multiple sizes:
$ convert favicon-16.png favicon-24.png favicon-32.png favicon-48.png favicon-64.png favicon.ico
Generator: http://www.buildmypinnedsite.com/
Tutorial: https://yoast.com/dev-blog/safari-pinned-tab-icon-mask-icon/
Clear Safari browser cached icons: ~/Library/Safari/Template Icons