--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/write_animation.py	Sat Mar 28 15:41:52 2020 -0700
@@ -0,0 +1,51 @@
+import bpy
+from pathlib import Path
+
+bl_info = {
+    "name": "Write animation",
+    "blender": (2, 80, 0),
+    "location": "Render > Write animation",
+}
+
+
+class WriteAnimation(bpy.types.Operator):
+    """Write a new webm next to the blend file"""
+    bl_idname = "render.write_animation" # Unique identifier for buttons and menu items to reference.
+    bl_label = "Write animation" # Display name in the interface.
+    bl_options = {'REGISTER'}
+
+    def execute(self, context):
+        if not bpy.data.filepath.replace('.', ''):
+            raise ValueError('Please save the file somewhere first.')
+        output_base = Path(bpy.data.filepath.replace('.blend', ''))
+        for num in range(1, 1000):
+            output_path = Path(f'{output_base}_{num}.webm')
+            if output_path.exists():
+                continue
+            break
+        else:
+            raise NotImplementedError()
+
+        render = context.scene.render
+
+        render.filepath = str(output_path)
+        render.image_settings.file_format = 'FFMPEG'
+        render.ffmpeg.format = 'WEBM'
+        render.ffmpeg.codec = 'WEBM'
+        render.ffmpeg.constant_rate_factor = 'HIGH'
+        render.image_settings.color_mode = 'RGBA'
+        bpy.ops.render.render('INVOKE_DEFAULT', animation=True)
+
+        return {'FINISHED'}
+
+
+def register():
+    bpy.utils.register_class(WriteAnimation)
+
+
+def unregister():
+    bpy.utils.unregister_class(WriteAnimation)
+
+
+if __name__ == "__main__":
+    register()