The new way to confirm ownership of a domain on Flattr is to add a meta tag to the page head. For example:

 <meta name="flattr:id" content="4j6y0v">

However, the meta directive doesn't allow setting of names with colons.

If I do this:

[[!meta  flattr:id="4j6y0v"]]

it gets rendered as:

<meta name="flattr:id=&quot;4j6y0v&quot;" content="" />

I tried a number of possibilities and they all failed to produce the correct output.

Directive syntax doesn't allow named arguments containing colons, so we would have to add a different syntax for weird names. However, we already have that:

[[!meta  name="flattr:id" content="4j6y0v"]]

This was missing from the documentation, but I have now added it. This feature was broken until 2015, but we now have an automated test to make sure it keeps working; the test includes a check for twitter:card which is essentially equivalent to what you're doing here. done --smcv