What is MD?
A Markdown table is the one people actually write by hand: pipes between the cells, a row of dashes under the header. GitHub, docs sites and half the world's READMEs render them, and their one real subtlety is that a pipe inside a code span is text, not a column break.
What is XLSX?
XLSX is what Excel has written since 2007 - not one file but a zipped folder of XML, with the sheet in one part and the strings in another. Its famous trap is dates: a date is stored as a plain number of days, and only the formatting attached to the cell says it is a date rather than a quantity.