What a plain-text field stores: the text a person typed, with every tag taken out and nothing escaped.

A title, a name, a short description or a notification's line is text, not HTML. It is stored as it was typed — Tom & Jerry's "cat" stays exactly that — and escaped where it is written out: HEEx escapes it into a page, MishkaCmsCore.Runtime.Seo into the head and its JSON-LD. Stored escaped instead, each of those escapes it a second time, and a reader sees Tom & Jerry.

Markup is taken out, never kept, by MishkaGervaz.Helpers.strip_tags/1: Tom <b>"cat"</b> is stored as Tom "cat", and <script>alert(1)</script>Hi as alert(1)Hi — the script's text, without the script. Removal repeats until there is nothing left to remove, so a tag hidden inside a tag (<<b>b>) goes too, and the result is stable: text(text(value)) == text(value), so saving a form twice changes nothing. An entity typed into a field is text like any other: & is kept as the five characters typed.

iex> MishkaCmsCore.PlainText.text(~s(Tom & Jerry's <b>"cat"</b>))
~s(Tom & Jerry's "cat")

On a field

A GuardedStruct field takes field/2 as its validator:, which runs before the field's derives:, so trim and the validate(...) ops see the text without its markup:

field :title, :string,
  validator: {MishkaCmsCore.PlainText, :field},
  derives: "sanitize(trim) validate(string, not_empty, max_len=255)"

Not a sanitize(...) op: an Ash resource's GuardedStruct pipeline only reaches the derive extensions the host application lists in its own config, and an op nobody listed passes the value through as it came. The validator needs no configuration, and a missing one fails compilation.