Spatial Navigation

https://forums.adobe.com/people/B+i+r+n+o+u  wrote yes, it is a real fun ground. many graphic arts students have implemented SPAs playing on such navigation (horizontal, vertical and transversal)... the visual interfaces are quite nice, a little confusing for a very corporate site but promising for the future Node (for developers) follows closely and there are many libraries both under famous used libs as react, angular, but also directly available directly under npm, ... in short there is enough to do your shopping under the various githubs tackling the subject without being linked to any framework

Your first statment, is possibly the biggest understatment I have heard for a long long time.

First, you can throw those libs in the bin, along with any thoughts of downloadable fonts and a simple 2x images being good enough, so font stacks ARE an essential once again.

Smart tv's have a bigger variation in what is, and what is not allowed or possible, than any mobile/desktop browser since NN4 and IE6, (or even the differences between a feature phone and a smartphone/desktop). Even the allowed total file sizes is controlled, (the trend of page weight getting bigger, is a no, video is a seperate subject and html5 essential) and js file size is a very big limiting factor, with some tv's being limited to (if js is allowed, thankfully only some older tv's appear to have js disabled) between 15 and 25kb in total, (not individual). On the other end of the scale js can be 500kb+, with testing on various tv's being the only way to check.

Android TV's are the easiest, (providing made within the last 3-4 years) but webOS from LG is also very popular, (and those browsers are not all created equal). Then comes 4k, (8k already appearing on the market) and don't forget line length can also be a problem on any tv above 40 inch, but an added problem is images as I said earlier, (srcset/picture/image-set is essential) which are better created using an actual measurement, (inches/cm/mm) and not pixel sizes, unless you really want to have a splitting headache every day creating them.

All that said, there is one very, very big advantage. In general people using 4k TV's do normally have fast broadband connections, so swapping content, and allowing high quality video is easy. Much more thought is required when developing for modern 4k tv's and game console browsers than any rwd mobile/desktop.

Always remember though, tv manufacturers, (and the browsers provided with) do try to compensate for the larger screen real estate, and that can catch the unwary developer out, more than the requirement for the viewport meta tag did at the beginning of the mobile development era.