Declarative support needs to be enabled in the HTML document by adding an attribute to the OpenUI5 bootstrap script tag.
This is done as follows:
data-sap-ui-modules="sap.ui.core.plugin.DeclarativeSupport"
OpenUI5 then requires (loads) the plugin sap.ui.core.plugin.DeclarativeSupport. When started, the plugin parses and enhances special HTML tags in the HTML document. The complete bootstrap script tag for OpenUI5 (based on a CDN version) looks as follows:
#!html <script id="sap-ui-bootstrap" type="text/javascript" src="resources/sap-ui-core.js" data-sap-ui-theme="sap_platinum" data-sap-ui-libs="sap.ui.commons" data-sap-ui-modules="sap.ui.core.plugin.DeclarativeSupport" > </script>