Parser function extensions, templates, and others may render error messages inline in pages. They have issues:
# They are usually in the user interface language but inserted into text in the page language which may
#* cause cache splits
#* bewilder users.
# They are styled inconsistently and individually, making CSSes unnecessarily bulky.
# Some may appear only on previews during section editing and not on the entire page, and vice versa.
# Since they may appear deep down somewhere, below a users will to scroll, they may go unnoticed by editor causing them.
So as to address some of these issues, let me suggest to collect these messages in a central place (container) which for page/section previews should be in the beginning of the area initially visible to the user.
We want to keep markers in the places where the errors are in the page, so make them use a bidirectional linking scheme similar or even identical to the one used for footnotes.
For normal page views, skins may decide where to render them. There needs to be only one central, skinable style to render the error messages, unburdening extensions and templates from dealing with local CSS and/or inline styles.
This suggestion was inspired by discussions on T114057 and the "balanced templates" RFC.