Support more HTML/CSS features via sinab rendering engine

[clauswilke]

The currently supported set of HTML/CSS features is extremely limited, and extending the supported feature set is an obvious priority for future development. This will require a new rendering engine, which is currently under development: https://clauswilke.com/sinab/

This issue serves as a catch-all for any feature requests that will be addressed by switching over to this engine. Features that are already known to work include:

• CSS selectors and external style sheets
• <pre> and code> tags
• Non-breaking spaces:  
• RTL text (with limitations, and only if graphics engine supports it)
• Improved vertical alignment options
• Setting margins, borders, box width/height from CSS