Output formats for IEC

Metanorma for IEC supports the general Metanorma output formats (html, doc, pdf), and also supports these IEC-specific output formats:

sts

NISO STS XML [added in https://github.com/metanorma/metanorma-iec/releases/tag/v1.4.3]

Note	IEC uses NISO STS in production.

metanorma -t iec -x html,sts test.adoc