Suggested change to description of merror layout in core spec

[NSoiffer]

The core spec says this about merror:

The merror element displays its contents as an ”error message”. This might be done, for example, by displaying the contents in red, flashing the contents, or changing the background color.

For the layout algorithm described in this document, the merror element must be treated the same as the mrow element. The user agent stylesheet must set some CSS properties on the merror element in order to highlight the error. As suggested in section 2.3.2, this can for example be achieved with the rule:

merror {
outline: solid thin red;
background-color: lightYellow;
}

I think it would be useful (and conformant with the HTML5 spec) to indicate what the error is when the content is not a string of text describing the error. This would occur when an merror is created because of a content error such as the wrong number of arguments to msup. I see two possibilities:

• Use the tittle attr so a tooltip comes up that explains the error ("wrong number of args to mfrac", etc).
• Use aria-describedby and point off to a description of the error

I prefer the first solution although tittle has some accessibility issues. Those will hopefully go away in the future as screenreaders adopt to current standards.

[fred-wang]

As I say on https://github.com/mathml-refresh/mathml/issues/15#issuecomment-469589832 I don't think we should rely on merror's rendering to indicate invalid markup. Also report of errors is generally done via the developer console.

Finally, I guess you are right that maybe merror is not in core, I opened #70 ; if merror stays in core, then I would oppose any additional complexity (I don't understand where the attributes you suggest are coming from? users? generated?). Even the current suggested rendering might not work until we clarify #9

[fred-wang]

I think this is something for an "invalid markup" polyfill, so adding the label

[davidcarlisle]

Closing as no change needed.