xml - Why is this XML header tag invalid?

Question

If I enter

<?xml version="1.0" encoding="utf-8"?>

into www.xmlvalidation.com it puts a cross next to xml and says "The processing instruction target matching [xX],mM][lL] is not allowed".

What is it complaining about?

TIA

Answer 1

它抱怨这个材料不是文件中的第一件事，所以它试图在 PI 而不是标题中解释。