This step was already done, although we have been working on a few improvements to how token and unknown elements are handled.
The <mmultiscripts> (multiple pre and post scripts) and <mpadded>
elements (tweaking spacing) have been implemented.
The only thing missing here is the basic implementation for MathML tabular elements, which has not started yet, we will consider it again in one or two months, depending on how fast patches for TableNG land.
We have implemented the low-level API to access the operator dictionary and parse operator attributes (form, boolean properties and spacing). Currently accent and accentunder attributes on underover scripts as well as largeop, stretchy and movablelimits attributes on the underover base are considered. The lspace and rspace attributes are now considered for formula spacing.
This was pending on the previous step and is now unblocked, we plan to start this important feature very soon!
As previously said, this is essentially done. We hope to discuss MathML Core related CSS properties this month with the CSSWG. Although this was not in the 2019 plan we also implemented new style for TeX’s “cramped superscript”.