Office Web Apps licensing offers two options:

• View-only. By default, Office Web Apps is view-only. View-only functionality is provided for free.
• Edit and view. You must purchase an editing license to use the editing features of Office Web Apps with SharePoint 2013. You enable editing when you create the Office Web Apps Server farm.

Enterprise customers who are licensed for Office 2013 through a Volume Licensing program can enable Office Web Apps editing for SharePoint 2013 on-premises. This helps make sure that users have Office editing capabilities at home or in other locations where Office clients might not be installed. Editing licenses for Office Web Apps are not available for separate purchase.

For exact details about your license, refer to the Microsoft Software License Terms that is shown when you install Office Web Apps Server.

SharePoint 2013 provides new license enforcement that works with Office Web Apps. If you enable SharePoint licensing and then enable Office Web Apps editing, only the users who have the appropriate license, which is known as OfficeWebAppsEdit, can actually edit Office files in a browser. If no Office Web Apps editing licenses are applied for users, only viewing is supported. For more information about how licensing works in SharePoint 2013, see Configure licensing in SharePoint Server 2013. The EditingEnabled parameter that enables editing is described in New-OfficeWebAppsFarm and Set-OfficeWebAppsFarm.

Read more here: Licensing Office Web Apps for editing Office files

Remember also, that in 2013 the Office Web Apps server/farm is a shared resource between SharePoint Server 2013, Lync Server 2013 and Exchange Server 2013.

Browser support, see image:

As you can see, owa now supports most common browsers

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