This was not a problem under LibreOffice 4x. Note-- possibly related to bug 65404??? I don't know enough tech to know for sure...
Not a problem with LibreOffice 4.2.8
Maybe when LibreOffice 5 changed how it does highlighting all the related and necessary changes didn't get made in the export to HTML module? https://wiki.documentfoundation.org/ReleaseNotes/5.0#Word-compatible_text_highlighting Word-compatible text highlighting Both highlighting and shading are preserved during import / export of Microsoft Word documents. New compatibility option to specify how to export LibreOffice character background to Microsoft Office file formats: as highlighting or as shading.
Can *not* confirm a loos of Character highlight. Text spans assigned highlight retain that background color on export/save. Rather the loss is with Paragraphs assigned a background color. But, means there *is* a loss of layout detail in that Paragraph background color is not coded into the export HTML 4, just the Character highlight. True for both Writer export to HMTL and Writer Web creation of HTML. Adjusting summary and setting New.
Sorry I didn't explain initially... in my case I highlighted in yellow one word of a sentence and then exported to HTML and the highlighting was gone from the HTML.... that was not a problem in the previous version I used (4.2.8).
*** This bug has been marked as a duplicate of bug 94088 ***