Add demos for things which required JavaScript on ancient browsers

[qgustavor]

Some features used to be implemented with JavaScript but now are supported natively. Sometimes those old implementations are slow and/or not mobile-friendly, so when possible would be good using the non-javascript versions. Some examples:

• position: sticky (demo, Can I use);
• animations (Can I use);
• placeholders (Can I use)
• media files (old implementations were flash based...)

[stevemao]

PR welcome