from: Oreilly.com

Including HTML Within a Feed

Whether the description element within RSS 0.92, 2.0, and 1.0 may contain HTML is a matter of great debate. In my opinion, it should not, for both practical and philosophical reasons. Practically speaking, including HTML markup requires the client software to have the ability to parse or filter it. While this is fine with many desktop agents, it is restricting for developers looking for other uses of the data. This brings us to the philosophical aspect. RSS’s second use, after providing headlines and content to desktop readers and sites, is to provide indexable metadata. By combining presentation and content (i.e., by including HTML markup within the description element), we risk
breaking this feature.

PS: 會引這篇是因為之前我發現對於同個 client 來說，它收一樣號稱 RSS2 的資料，有的網站會把 HTML tag 拿掉，有的不會。這篇或許提供了一些想法。