Here is an explanation about html and e-mails, I suppose it is the same for your logs?
http://www.viruslist.com/en/weblog?weblogid=154618750
It is recommened to use plain text
In another link found this explanation:
It is possible for HTML files to contain malicious code or instructions for the browser to run a remote piece of code. It could be a JavaScript, a VBscript, a link to a remote Java applet, or a link to a remote ActiveX control.