This Issue is connected with https://jira.atlassian.com/browse/CONF-30725. In our lozenge and clickable macros we have defined link parameter with type "confluence-content".
In previous versions of Confluence it was possible to put into this parameter both:

• link to another Page in Confluence instance
• URL (for example: http://www.adaptavist.com).
  In Confluence 5.3+ when we put URL as a input - The macro fails to render and displays a red warning icon with the following tooltip:

  Unhandled element type '{http://www.w3.org/1999/xhtml}a'

  To reproduce issue I created very simple macro that show confluence behaviour of Confluence for parameter with type "confluence-content". (See the attachment)

There is workaround - to change parameter confluence-content type into string type, however we expect that version Confluence 5.3 should let us use the same type as we were using in previous versions.