Mobile webapps should be based on the requirements and guidelines mentioned in the following W3C documents:
Web technologies are a fast moving target, so rather than tying these requirements to specific web technology versions (HTML, DOM, CSS etc.), please refer to the good practices mentioned in the above documents.
WAP 2.0 devices should support XHTML Basic 1.1, (WAP 1.0 was based on WML) and CSS Mobile Profile 2.0.
Guidelines and requirements for native apps should be based on the market shares of various platforms in Estonia and elsewhere, and also somewhat on the forecast of future shares.
From a security perspective, operating system requirements should always follow the latest version (e.g., Android 5.x, iOS 8.x, Ubuntu Touch OS 14.x etc.).