Complex sentence: As a document markup language, SGML was originally designed to enable the sharing of machine-readable large-project documents in government, law, and industry, that must remain readable for several decades--a long time in the information technology field.
Split into simpler sentences: As a document markup language, SGML was originally designed to enable the sharing of machine-readable large-project documents in government, law, and industry. Many of these documents must remain readable for several decades--a long time in the information technology field.