Comment 3 for bug 744819

a) Element 'FlowRoot'/'FlowPara' undefined

(…)
   version="1.2"
(…)
  <flowRoot id="flowRoot4080" (…) >
    <flowRegion id="flowRegion4082">
      <rect id="rect4084" (…) />
    </flowRegion>
    <flowPara id="flowPara4086" />
  </flowRoot>

Non-compliant implementation of 'Flowed Text' - duplicate of
Bug #167335 “flowed text has no SVG 1.1-compliant fallback”

Empty, invisible 'Flowed Text' - duplicate of
Bug #485269 “Invisible and unselectable flow region in image”

b) References to text styles (including font family)
These are not invalid as far as I understand, and (IIRC) originally kept on purpose as style attribute of the resulting path or parent group after converting text to path - possibly to be able to reconstruct the converted text object or add more text using the same font specification.

See issue 2) in bug #390549, as well as bug #171983

c) -inkscape-font-specification
This attribute probably should not be in the "Plain SVG" file, though as CSS attribute I'm not sure it is invalid in a strict sense (the question whether a separate attribute within the inkscape namespace should be used instead was briefly discussed in bug #169973).

Please do not forget to always provide information about your Inkscape version and OS/platform used to reproduce the reported issue.