Google announced the launch of a new tool to help novice webmasters create rich snippet markup (structured data) for their web pages. The tool is a point-and-click tool that lets a webmaster click on words or phrases and add elements of structure data to it without having to go into the code and add it to the HTML.
Here is how Google describes it:
Data Highlighter is a point-and-click tool that can be used by anyone authorized for your site in Google Webmaster Tools. No changes to HTML code are required. Instead, you just use your mouse to highlight and "tag" each key piece of data on a typical event page of your website.
It looks easy, I have not tried it because of a couple reasons:
(1) I have HTML based structured data markup on most of my sites that need it.
(2) It is only available right now for event based data.
That being said, someone asked, what if you have both HTML markup and then also use this tool to add markup. What does Google decide to use?
John Mueller answered this on Google+ saying:
To a large extent, I think SEO is about giving a clear & consistent signal to search engines. As soon as you give inconsistent signals, - or worse: conflicting signals - then you can't rely on them doing what you'd really want them to do. Sometimes that's obvious (specify a URL in a Sitemap file that redirects elsewhere), other times it's subtle (specify one element of a page as the name of an event, and use the Data Highlighter to tag a different element as the name). Essentially, it's up to you to provide clear & consistent signals if there's something specific that you want search engines to do.
The best case, someone should try to test it and see what Google decides to use. Let us know.