The Core Filing offering only allows uploading of files, whereas Free Formatter allows either pasting of documents or pointing to URLs (but not uploading files! Both work fine and seem to use the Apache Xerces parser underneath the hood, which I’ll talk more about in the next section.
The problem with on-line validation is that it will quickly get frustrating if you are doing a lot of validating and need a quick turn-around time. For Mac and Linux users, you likely already have xmllint installed, which can do validation.
There are actually a few different options at your disposal (which is probably part of the problem).
We’ll start by looking at what is probably the fastest and easiest option to get started with validation: A web search reveals the Core Filing XML Schema Validator and the Free Formatter Validator amongst others.
Also, the validators vary in quality of error messages; if you get stuck it is often worth running against a different validator to see if the error message is more helpful.