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>