https://zzz.buzz/zh/notes/case-sensitivity-in-html/

HTML 标准规定所有的字符串除特别说明外都是要区分大小写的....

https://www.w3.org/TR/html5/single-page.html#case-sensitivity-and-string-comparison

html 屬性 ID 和 Class 的值是區分大小寫的

But HTML class names are case-sensitive