HTML-formatted postings are not welcome in newsgroups. While your software may interpret HTML correctly, many news-reading clients have only minimal ability to display HTML correctly. In addition, HTML postings often include your message twice: a plain text version is followed immediately by a version including HTML tags. This extra version of your message more than doubles the total length of the posting, and adds unnecessary download time for other newsgroup readers. Between the inability of other users’ software to interpret your posting correctly and the longer postings costing additional money to download (much of the world uses dial-up connections, but does not have free local calling), you’re likely to receive complaints if you post in HTML.
If you post with vCards, you will likely also get requests to stop doing so. For those using other newsreaders, vCards just look like a long signature, which newsgroup readers commonly find annoying.