OP's query is on the omission of the article:
Like the sentences in the attached link, they did not put articles before "Chair has" or "Seat is".
Why do they omit articles?
This is an example of "zero article" usage. Zero article refers to the absence of articles (the, a, an) before nouns.
There are a number of rules where we don't use any article and it is given in the shared link.
The checklist is from Occupational Safety and Health (OSHA).
"This checklist can help you create a safe and comfortable computer workstation.".
- Backrest has height adjustability so support is provided for the lower back (lumbar area).
- Chair has a sturdy 5 leg base.
- Seat width and depth should accommodate the specific user (seat pan should be wide enough for ease of egress and deep enough to support the entire thigh but not so deep that user cannot utilize lumbar support.)
The checklist by OSHA follows the guidelines since it is a technical instruction related to improving the "Ergonomics" (relating to the design of furniture or equipment which makes it comfortable and effective for people who use it.).
Reference englishclub.com.
One of the rules for Zero article states that
ZERO Article::
Newspaper Headlines, Notices, User Guides, instructions,
https://www.englishclub.com/grammar/determiners-zero-article.php
Zero article helps to focus on essential information and creates a concise, objective tone.