rss/markup: added validators

wiki/markup_language.md

@@ -0,0 +1,18 @@
+# Markup Language
+
+A markup Language is way to encode text so that it has great amounts of structure and formatting.
+The most common markup languages are [HTML](#html), Markdown, LaTeX and [XML](#xml).
+
+## HTML
+
+HTML is a markup language that is mostly used in the internet via the hypertext system and accessed
+via [web browsers](/wiki/web_browser.md).
+For styling HTML accordingly CSS is used most of the time.
+HTML can be quite challenging to write, read and check for validity.
+An easy way to do the latter one is a validation service.
+One of the most used ones is the
+[Markup Validation Service by W3C](https://validator.w3.org/#validate_by_input).
+
+## XML
+
+XML is a markup language often used together with [RSS Feeds](/wiki/rss.md).

wiki/rss.md

@@ -11,3 +11,10 @@ This sections lists various RSS clients.
 - [FreshRSS](/wiki/freshrss.md) is a self-hosted RSS aggregator with a web interface.
 - [Feeder](https://f-droid.org/de/packages/com.nononsenseapps.feeder/) is an RSS reader for
   [Android](/wiki/android.md) phones available on [F-Droid](/wiki/android/f-droid.md).
+
+## Validation
+
+RSS and XML are not very human-readable like other [Markup languages](/wiki/markup_language.md).
+For this a validator can be used.
+One of the most popular ones is the
+[Feed Validation Service by W3C](https://validator.w3.org/feed/#validate_by_input).