This is in reply to your 10/27 post 3 other Psalms questions in regard to Psalm 96:9a and your following question:

Psalms 96:9a O worship the LORD in the beauty of holiness:

Does, "in the beauty of holiness", only mean the LORD's holiness? Or can it also refer to the worshipper, i.e. worshiping in holiness?

The link at Our Beautiful God states the following about Psalm 96:9a:

"This passage conjoins the holiness and glory of God with respect to the idea of beauty. We are called to come into the presence of God and to worship what is beautiful about Him—His glory and holiness."