How to prevent strechtability?

[David Carlisle]

the only stretchability there is \parskip which has the default

\parskip=0pt plus 1pt

that 1pt is then over stretched (tex warning on the console)

You could set \parskip to 0pt and make the page even more underful but
perhaps you want

\raggedbottom

so stretch glue is added at the end of the page, so \parskip isn't
forced to stretch.

(ignoring the fact that the example drops all non ascii letters if
used with classic tex)

David