XML Formatter indents nested elements, keeps comments and the declaration on their own lines, and does not add depth for void or self-closing tags.
It is not a validating parser, but it points out obvious tag-nesting problems. It runs in your browser.
No. It formats what it can and warns about unclosed or mismatched tags, but it will not reject invalid input.
By default it collapses runs of whitespace in text nodes. Significant whitespace, as in a pre block, is not preserved.
Roughly. Void elements like br and img are handled, but HTML's optional closing tags are not, so results are best with well-formed markup.
No. The tool runs entirely on this page.