This project shows a "valid swagger" badge on your site. Temporarily, there is an online version hosted on the Reverb petstore sample.
You can validate any swagger specification against the Swagger 2.0 Schema as follows:
<img src="http://online.swagger.io/validator?url={YOUR_URL}">
Of course the YOUR_URL needs to be addressable by the validator (i.e. won't find anything on localhost). If it validates, you'll get a nice green VALID logo. Failures will give an INVALID logo, and if there are errors parsing the specification or reaching it, an ugly red ERROR logo.
For example, using https://raw.githubusercontent.com/swagger-api/swagger-spec/master/examples/v2.0/json/petstore-expanded.json as a source, we get ...
If your specification fails to validate for some reason, or if there is an error, you can get more information on why by visiting http://online.swagger.io/validator/debug?url={YOUR_URL}.
Since the validator uses a browserless back-end to fetch the contents and schema, it's not subject to the terrible world of CORS.