In some versions of the theme, there's a minor bug which has been fixed in recent versions and is an easy fix. If you're seeing a <span> message when a variant is sold out, here's how to fix it (fix time: 1 minute).
- Go to Admin > Themes > edit html/css
- Open snippets/product-scripts.liquid
- Find "text: message" (use CTRL+F to find it quickly)
- Change to "html: message"