Hi @Gloria Dangerfield ,
Thank you for posting to Microsoft Community.
Based on your description, I understand you want to match some HTML tags in Except If conditions.
However, you could add these HTML tags in your disclaimer, but I’m afraid use HTML in the conditions is not supported yet.
If you want to avoid multiple disclaimers being added in an email conversation as you mentioned, you could add an exception that looks for other sections of the disclaimer that are not dynamic html tags in your disclaimer. This exception ensures that the disclaimer is only added to the original message.
You could refer to Organization-wide message disclaimers, signatures, footers, or headers in Exchange Online | Microsoft Learn for more information.
Hope the information above helps and thanks for your understanding.