Improve layout on smaller screen widths / print layout

Make floating body be opt-in for screens with width >= 1024px.
Make floating screenshot next to main text content opt-in for screens with width >= 1500.
Screenshot crushes the content width so it wraps the text a lot, so only done when very wide.

Also moved from duplicate styles into stylesheet file.
Fix a few HTML issues reported by Dillo web browser
Improve title text for alt text for miku and screenshot