Prayer books contain the words of structured services of worship - the prescribed processes & rituals, as set out by the Church, to be followed by the priest and congregation for various services, e.g. weddings, communion, etc.
Not following the liturgial steps exactly as spec'd out was not thought of as a minor slip-up - each was considered fundamental to proper worship. So, if a priest wasn't willing or able to follow the authorized script, then that priest was not going to be certified as being on the approved 'Avoiding Everlasting Hell Guidance Providers' list. (at the time, the consequences of not being on the list were not inconsequential.)
If however, a priest was on the list (as evidenced by his not being engulfed in flames) then those members of the congregation who relied on his guidance knew exactly what they were getting - there would be no 'Well I wonder what he will talk about today'. You could say that they had complete assurance in the priest and processes.
Sounds like NIST 800-63, but with more incense.
What would it have looked like if it used RFC 2119?
When there are children to he baptized upon the Sonday, or holy day, the parentes SHOULD geve knowledge over nighte, or in the morning, afore the beginning of Mornyng prayour, to the curate. And then the Godfathers, Godmothers, and people with the children, MUST be ready at the Fonte, eyther immediatly after the last Lesson at Morning Prayour, or els immediatly after the last Lesson at Evening Praiour, as the Curat by his discretion shall appoincte. And then standinge there, the Priest SHOULD ask whether the children be Baptized or no. If they aunswer, No. Then the priest MUST say thus.
DERELY beloved, forasmuche as al men be conceived and borne in synne, and that our saviour Christ saith, none can entre into the kingdom of God (except he be regenerate, and borne a new of water and the holy gost) I beseche you to cal upon God the father, throughe our lord Jesus Christ, that of his bounteous mercy, he wil graunt to these children, that thing which by nature thei can not have, that they may be baptized with water and the holy Ghost, and received into Christes holy church; and be made lively membres of the same.
Then the Priest MUST say.
Let us praye.