It seems to me that chameleon does not behave properly with boolean attributes:

[ale@flo chameleon]$ cat test.pt
<input required="" tal:attributes="required string:required">
<input required tal:attributes="required string:required">
<input required foo="bar" tal:attributes="required string:required">

[ale@flo chameleon]$ cat test.py
from chameleon import PageTemplate

with open("test.pt") as f:
    print(PageTemplate(f.read())())

[ale@flo chameleon]$ python test.py 
<input required="required">
<input requiredrequired>
<input requiredrequired foo="bar">