HTML rendering programs have been very forgiving with developers.
If you forget to close a tag, the closing tag is usually inferred.
If you use the wrong tag, the browser disregards it and renders the document, no matter how ill-constructed it is.
For that reason, HTML is great for the lazy developer. Half an effort can still produce attractive pages.
XML, on the other hand, is far stricter. Any single error will prevent your page from rendering. XML is not for those who dislike detail or want to construct pages haphazardly. XML requires attention and discipline.