This may look like HTML, but it's really just Rich Text. It goes in the ArcGIS Online Homepage of your ArcGIS Online Organization. See This Page for more information and examples. http://doc.arcgis.com/en/arcgis-online/reference/supported-html.htm
I have simply copied these examples and customized them for our own ArcGIS online Organization. http://cloudpointgeo.maps.arcgis.com/home/index.html