The following small piece of code yields $Failed when executed in a cloud notebook through a browser.
However it gives the proper answer when executed in the same cloud notebook accessed via the “Wolfram Desktop” (and, of course, works in a desktop Mathematica environment):

temp=ExportString[(1+a)/b,"MathML"]; 
ImportString[temp]

Any thoughts on this? I've reported it as a possible bug, but would like to understand whether it is by design and, if so, what are the additional restrictions on cloud programming--whether in cloud notebooks, Instant APIs, or cloud Forms.