Fix HTML output #18292
Description
Html output also suffers from the EMBEDDED_MODE syndrome, it should actually return html expressions (instead of the empty string) and not rely on print()ing HTML to the screen doing anything.
The current behavior is:
sage: n=7
sage: L = map(None,[p for p in prime_range(n+1) if p%4==1],[p for p in prime_range(n+1) if p%4==3])
sage: L = [['',l[1]] if l[0] is None else l for l in L]
sage: T = [['$p\equiv 1\\text{ mod }(4)$','$p\equiv 3\\text{ mod }(4)$']]
sage: output = html(table(T+L,header_row=True)) # print() to stdout???
<html>
<div class="notruncate">
<table class="table_form">
<tbody>
<tr>
<th><script type="math/tex">p\equiv 1\text{ mod }(4)</script></th>
<th><script type="math/tex">p\equiv 3\text{ mod }(4)</script></th>
</tr>
<tr class ="row-a">
<td><script type="math/tex">5</script></td>
<td><script type="math/tex">3</script></td>
</tr>
<tr class ="row-b">
<td></td>
<td><script type="math/tex">7</script></td>
</tr>
</tbody>
</table>
</div>
</html>
sage: repr(output) # what?
''
sage: type(output)
<class 'sage.misc.html.HTMLExpr'>
CC: @novoselt @kcrisman @jhpalmieri
Component: notebook
Author: Volker Braun
Branch: 553d2bb
Reviewer: Andrey Novoseltsev
Issue created by migration from https://trac.sagemath.org/ticket/18292