There’s an updated (June 2016) “Licensing Microsoft Office software in Volume Licensing ” VL Brief which is updated for Office Online Server. This is the successor product to Office Web Apps Server 2013 and provides browser-based versions of Word, PowerPoint, Excel and OneNote from on-premises environments.
For details on the licensing, the FAQs on page 8 are useful, but in summary: there are no licences required for viewing documents, but create/edit/save rights are an SA benefit and included in an Office 365 ProPlus subscription.
However, there’s an exception which means that licences bought without SA before 1 August 2016 don’t need SA before 1 August 2019.
Find the guide in the Desktop section of our Licensing Guides emporium: http://bit.ly/MSLicensingGuides.