Here's an example from Python:

>>> import minify_html
>>> minify_html.minify('<iframe sandbox=""></iframe')
'<iframe></iframe>'
>>> minify_html.minify('<iframe sandbox></iframe')
'<iframe></iframe>'
>>> minify_html.minify('<iframe sandbox="true"></iframe')
'<iframe sandbox=true></iframe>'

An empty value is meaningful here, quoting MDN:

sandbox
Controls the restrictions applied to the content embedded in the <iframe>. The value of the attribute can either be empty to apply all restrictions, or space-separated tokens to lift particular restrictions.

This changes the behaviour of the iframe, because now there are no restrictions applied to the contents of the iframe. This is potentially a security risk, as a previously sandboxed iframe is now unrestricted.