Schematron is a language for validating XML, designed for processing XML documents and reporting on errors. Version 1.0 was developed in 1999, since when it has been enhanced and standardised, with the latest version being ISO/IEC 19757-3:2020. This replaced the 2016 version: ISO/IEC 19757-3:2016.
Last month Jelliffe, who is no longer directly involved in editing the standard, posted to "note with extreme displeasure that ISO/IEC has recently reneged on its policy of making available free PDFs for standards that were brought into ISO from the outside: they want you to buy it.
"In the case of the 2020 edition, which is some corrections and additional annexes only to the freely available 2016 edition, it is ridiculous bastardry on ISO's part."
Jelliffe said that "the core library technologies are almost entirely implemented or maintained by private individuals as open source projects (such as David Maus' SchXslt): these small or not-for-profit developers should not have to abruptly fork out $175 for the 2020 edition, which differs only in a few pages from the freely available 2016 edition."
My comment is I'd really hope that open standards that we want universally adopted, should be freely available, at least to open source developers whose projects are usually not commercial by nature.
See Open standard but not open access: Schematron author complains about ISO paywall
'This is shooting Schematron in the heart ... its heart is individual open source developers'