The proper Yaml syntax for a list of tags would be either:
tags: ['#tag_one', '#tag_two']
or
tags:
- '#tag_one'
- '#tag_two'
The quotes are necessary because # has a special meaning in Yaml as the start of a comment, as @bdillahu already noticed.
However, Obsidian (in recent versions) does two things to make this less cumbersome:
- It adds the
# automatically, so you don’t need to specify it, and then you don’t need the quotes either.
- If you don’t specify a list in Yaml, then a single Yaml value is split automatically using blanks and/or commas as separator.
So, utilizing these two features of Obsidian, you can just write:
tags: tag_one tag_two
The only small problem is that Obsidian (currently) still doesn’t show suggestions while typing the tags that way. As a workaround, you can add the tags with a starting # so that Obsidian suggest possible tag names and then remove the ‘#’ again to make it proper Yaml syntax. If there is a Yaml syntax error, then Obsidian will not read the frontmatter.