Reposted from Groundspeak Forums:

On Tuesday the 17th of January a new HTML filter will be implemented on the site which strips certain tags, attributes and values from cache descriptions which we have found to be of particular concern for the security of users. After examining all the active caches in the database we have identified around 400 cache owners who will be directly affected by the change. As a courtesy to those cache owners we have setup this forum thread so owners can collaborate and discuss workarounds for the upcoming change.

All users owning one or more caches containing soon-to-be-filtered code will receive an email.

For full post/details please read the post on Groundspeak forums