For those that wish to include xhtmlrenderer, I've reviewed clause 4 of LGPL v3, which says you must do each of the following:
a) Give prominent notice with each copy of the Combined Work that the Library is used in it and that the Library and its use are covered by this License.
The docx4j NOTICE file nearly does this; it should specifically say v3
b) Accompany the Combined Work with a copy of the GNU GPL and this license document.
Not hard to do.
c) For a Combined Work that displays copyright notices during execution, include the copyright notice for the Library among these notices, as well as a reference directing the user to the copies of the GNU GPL and this license document.
This one is up to you, if applicable.
d) Do one of the following:
0) Convey the Minimal Corresponding Source under the terms of this License, and the Corresponding Application Code in a form suitable for, and under terms that permit, the user to recombine or relink the Application with a modified version of the Linked Version to produce a modified Combined Work, in the manner specified by section 6 of the GNU GPL for conveying Corresponding Source.
1) Use a suitable shared library mechanism for linking with the Library. A suitable mechanism is one that (a) uses at run time a copy of the Library already present on the user's computer system, and (b) will operate properly with a modified version of the Library that is interface-compatible with the Linked Version.
Go with option 4d1, since Java jars are a "suitable shared library mechanism"; see http://www.gnu.org/licenses/gpl-faq.html#LGPLJava
e) Provide Installation Information, but only if you would otherwise be required to provide such information under section 6 of the GNU GPL, and only to the extent that such information is necessary to install and execute a modified version of the Combined Work produced by recombining or relinking the Application with a modified version of the Linked Version. (If you use option 4d0, the Installation Information must accompany the Minimal Corresponding Source and Corresponding Application Code. If you use option 4d1, you must provide the Installation Information in the manner specified by section 6 of the GNU GPL for conveying Corresponding Source.)
This bit about "Installation Information" is as clear as mud. I can't see any clear requirement to provide the source code for xhtmlrenderer, which is a bit surprising, but it would probably be simplest just to package it in the xhtmlrenderer jar.
In summary, I don't see any reason why someone wishing to include xhtmlrenderer in their commercial application wouldn't do so, but again, you should make your own decisions based on your own legal advice.