It's a Byte Order Mark, , which is invisible to most text editors. Try using VIM and killing it or somehow find a different text editor and kill everything before <!DOCTYPE html>
You can simply copy <!DOCTYPE HTML> and everything below it into a new file and save over it, meaning you wont be copying the BOM coming before it to resolve this issue; I've ran into it many times.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
