user29432: When serializing a PSBT (BIP-174, Partially Signed Bitcoin Transaction), I cannot find any information if the keys should be ordered when serializing the PSBT?
Under the "Test Vectors" section for the BIP-174 in the last example it is only mentioned that:

A combiner which orders keys lexicographically must produce the
  following PSBT[...]

But it does nowhere say that it is mandatory to order the keys. What is expected?
I am implementing test cases for a PSBT library and would like to know this to decide if I should compare the serialized PSBTs (where ordering does matter) or the objects themselves (regardless of ordering of keys). 

user48884: There is no mandatory ordering. If you assume your parser is correct, then it is fine to compare the objects themselves. However, to be sure everything is correct, I would suggest that you compare the serialized PSBTs themselves.

29432: <p>When serializing a <a href="https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki" rel="nofollow noreferrer">PSBT</a> (BIP-174, Partially Signed Bitcoin Transaction), I cannot find any information if the keys should be ordered when serializing the PSBT?</p>

<p>Under the "Test Vectors" section for the BIP-174 in the last example it is only mentioned that:</p>

<blockquote>
  <p>A combiner which orders keys <em>lexicographically</em> must produce the
  following PSBT[...]</p>
</blockquote>

<p>But it does nowhere say that it is mandatory to order the keys. What is expected?</p>

<p>I am implementing test cases for a PSBT library and would like to know this to decide if I should compare the serialized PSBTs (where ordering does matter) or the objects themselves (regardless of ordering of keys). </p>
 48884: <p>There is no mandatory ordering. If you assume your parser is correct, then it is fine to compare the objects themselves. However, to be sure everything is correct, I would suggest that you compare the serialized PSBTs themselves.</p>
