> The spec is a beast. Anyone who wants to implement an HTML based reader has a mammoth task in front of them.
That's true for basically any non-trivial document rendering format. For example, take a look at the PDF spec. Even basic things like parsing the document format is a formidable task. HTML in comparison is a trivial format. The same goes for technologies like TeX or even Microsoft's own Word format, which Microsoft famously had lots of problems supporting. It is a hard problem for all formats, not just HTML.
> The point of the thread was to tell how big of a task it is to create your own HTML based reader.
You're confusing some things. A document format is one thing, but a renderer with specific capabilities is an entirely different thing. You're commenting on the document format and the styling and layout system, and now you're shifting the conversation to what it takes to implement a renderer.
Debates on document formats are entirely separate and orthogonal to debates on how to implement renderers. Renderers for the most trivial things are tremendously complex. There are a myriad of good reasons why we're seeing GUI frameworks built on top of webviews in spite of all the complains about the formats that webviews support, and in spite of the myriad low-level rendering frameworks already available.
To understand the poiny, try to think through the requirements list to implement a renderer for Markdown. It's a document format with a half dozen of features. Would you call it trivial?