(Broken off from T92969: VE in IE is stripping the contents of <font> tags from pasted text):
When pasting two words, one plain, one bolded from MS Word Online into IE, the console.log shows the following:
Before sanitation: <p data-parsoid='{"dsr":[61,182,0,0]}'>☀<span class="TextRun
SCX167197419" xml:lang="EN-US"><span class="NormalTextRun
SCX167197419">plain </span></span><span class="TextRun SCX167197419"
xml:lang="EN-US"><span class="NormalTextRun SCX167197419">bold</span>
</span></p>(Line breaks added to improve readability.)
This is a little different than what http://edg2s.github.io/content-editable-sandbox/ shows since the style attribute seems to be stripped instead of interpreted: https://gist.github.com/hexmode/81988a6ad474c5c51553
Plain text appears in VE.
The style elements of the paste should inform VE that one of the <span>s need to be turned into <b>.