It's a very common Markdown extension supported by PHP-Markdown-Extra, Goldmark, Pandoc, Kramdown, and dozens of others. Several of these have supported it for almost ten years now, with the same syntax.
PHP-Markdown-Extra is the closest thing to a standard with more than GitHub-Flavored-Markdown; several other Markdown engines use its featureset as a baseline for compatibility for anything not present in GFM, even blocking the shipping of new features until after PME agrees on a syntax. So you can think of CommonMark as the lowest common denominator, GFM for an intermediate version, and PHP-Markdown-Extra as something suitable for building more advanced websites.
PHP-Markdown-Extra is the closest thing to a standard with more than GitHub-Flavored-Markdown; several other Markdown engines use its featureset as a baseline for compatibility for anything not present in GFM, even blocking the shipping of new features until after PME agrees on a syntax. So you can think of CommonMark as the lowest common denominator, GFM for an intermediate version, and PHP-Markdown-Extra as something suitable for building more advanced websites.