The path to the XSD file is: {view.request.baseUri}{xsdPath}. Please use the following snippet for autocompletion:
<?xml version="1.0"?> <html xmlns="http://typo3.org/ns/html-base" xmlns:xsi="http://typo3.org/ns/html-base" xmlns:{namespacePrefix}="{namespaceUri}" xsi:schemaLocation="http://typo3.org/ns/html-base {view.request.baseUri}Resources/Packages/Fluid/html-base.xsd {namespaceUri} {view.request.baseUri}{xsdPath}"> <!-- Your content goes here --> </html>