W3C Resources
I am impressed! There is ALL you need to design a great website is on the W3C Schools website. Well, at least that is the way that it appears. Sure, there is a revenue stream here as they offer certification services for a fee, but the information is free. The only thing is that you have to find it.
Social Media Buttons
As Social Media is s pervasive today many will want to add buttons or other navigation to their sites. W3C Schools show you how to add "sticky" buttons and menus to your website. This would be particularly useful if your "visitorship" access your site on a mobile device and use Facebook, WhatsApp or other social platforms.
Image Gallery
I have tried this in the past but I had not approached it from a css perspective. The W3C Schools has a few examples that I will/have experimenred with.
My examples are taken from the W3C site and include: CSS Flexbox Responsive Image Grid Slideshow
CSS Flexbox
The CSS Flexbox technique is used in the Responsive Image Grid example. The technique can also be used to format web pages in a responsive manner, See the example Responsive Website using Flexbox