plainformat (not XeLaTeX).
The differences between Plain TeX and XeTeX are fairly minor from the point of view of markup. XeTeX extends the TeX
\font primitive so you can use modern outline fonts (PostScript, TrueType, OpenType) without having to go through complicated set-up routines such as writing your own .tfm files.
It also supports Unicode input, so characters such as £ Æ ð Ħ ő € αβγ абв etc can be entered directly in the source file without any special markup. XeTeX files are assumed to be in UTF-8 encoding by default, in contrast to Plain TeX which uses ASCII.
The pages below cover various aspects of typesetting with (Xe)TeX in greater detail.
(Neocities won’t let me upload .tex files, so I have uploaded them as UTF-8 plain text files. But if you paste them into your TeX editor and save them and run them they should work.)
Chapter, Section, Subsection