Yes, I saw that reference, but I misunderstood. If you are validating the HTML then that's a nice validator, but you really can't validate XSL or XML with it to debug an xsl. Even though XML/XSL is like HTML, there are some other considerations. That's why I mentioned using Xselerator, which is an XML/XSL editing tool, you would find really useful if you want to actually learn more about what's going on, and how to start customizing XML or newsfeeds.
Even Visual Studio is helpful somewhat, but I find Xselerator to be more useful, and powerful. The method I use, is to create the newsfeed, then have it rendered on a page. Then, I view the source, and copy the text into Xslerator, which allows me to view the code, and tidy it up. You can also, submit the rendered newsfeed to another validator that will give you information like the HTML validator you are using, but it's for validating newsfeed. http://validator.w3.org/feed/
Now, once I have an RSS v2.0 valid newsfeed, which is a real requirement with the News v4.x module, I'll take that source and place it into Xselerator as the Input XML. Then, I can use whatever XSL I'm working on and use it as the Input XSL file. This setup allows me to test the changes I'm making to the XSL against a good newsfeed. You can open up several XSL files to test against the source XML. There is some additional setup to mimic the DNN News module rendering (up to a point), and that involves setting up the parameters for the variables used in the News module to set up the rendering, items to show '-1' (allows all items to be shown), show item details 'true' (allows all item info to display - like description rather than just title), and show item date 'true' (allows the pubDate to display). Now, you can test via Xselerator by pressing f5 or the green arrow button to render the XML using the active XSL you are testing in a browser. You'll have move options using Xselerator to test and develop as you learn more.
That process gives me pretty good results in developing custom XSL's for the News module. I can start with the default.xsl in the transformations folder and save my modified XSL to the portals/n folder for importing and use in the module as I need. Errors in the XML or XSL are flagged, but you have to really inspect your code, because the flagged error may not be the only error, as I said above. Xselerator is available as an open source download, just google on it to find it. It's really worth the trouble to learn and use it, if you are getting serious about developing custom XSL's.
It seems like maybe the issue you had was being caused by some other HTML tag that was not closed, rather than the <br/> tag. It's confusing to debug at once, which is why I take the additional steps of generating the news feed and checking as I mentioned. It works for me, anyway, and bottom line is do what works for you.