Text Rendering Hates You
Rendering text, how hard could it be? As it turns out, incredibly hard! To my knowledge, literally no system renders text "perfectly". It's all best-effort, although some efforts are more important than others.
I'll be assuming you want to support arbitrary text provided by users with custom fonts, colors, and styles with line-wrapping and support for text-selection. Basically the minimum required to properly display a simple rich-text document, terminal, webpage, or anything else.
The overarching theme here will be: there are no consistent right answers, everything is way more important than you think, and everything affects everything else.
The topics I focus on here have no particular rhyme or reason, they're just the ones that come to mind after a few years of working on rendering in Firefox. For instance, I don't spend much time talking about the challenges of text-segmentation or managing the different platform-specific text libraries, because I don't look at that much.
Text is complicated and english is bad at expressing these nuances. For the purpose of this document, I will try to stick to the following terms. Note that these words aren't "right", I just find them useful for communicating the key concepts to native english speakers who don't have backgrounds in linguistics.
