Error: CSS: margin-block-start: Property margin-block-start doesn't exist.

URL being validated or code to reproduce error:
https://w3c.github.io/i18n-glossary/

|Error: CSS: margin-block-start: Property margin-block-start doesn't exist. |

margin-block-start and numerous other -block-start and -block-end 
property pairs do exist and work in browsers. Furthermore, their use is 
strongly recommended for internationalization.

Please fix.

For a few more examples of related things that should work, see 
https://www.w3.org/International/questions/qa-html-dir#alignment