The thing most people find ostentatious about the "mobbaroque" examples is that gold is used to add gold to the work. As a physical demonstration of wealth, i.e. "I can afford so much gold it's all over my artwork". The reason people find the Byzantine halos less ostentatious is that gold is used symbolically in lesser quantities to represent something else which is perceived as valuable.

This still doesn't establish why one should regard gold as "ostentatious". Are you working from some private definition of the term? I make use of the common pejorative meaning, given by Merriam Webster as:

  attracting or seeking to attract attention, admiration, or envy often by gaudiness or obviousness : overly elaborate or conspicuous : characterized by, fond of, or evincing ostentation

The key to ostentation is vulgarity or crudeness, ugliness. You even use the word "tacky", so clearly you aren't blind to at least the general idea of bad taste, even if we may be in disagreement about what constitutes bad taste.

Obviously, gold can be used to signify the importance of something, but that doesn't make it ostentatious. What I contest is the pejorative.

> that’s a judgement about what should be venerated not the intent behind the use of gold.

The use of gold is a matter of intention. A judgement is made that something is worthy of honor, another that gold is an aesthetically suitable material that, in some way, can be used to assist in honoring said thing, and the intention is made to honor said thing in such a manner.

> The key to ostentation is vulgarity or crudeness, ugliness.

That’s not in the definition you quoted. The 2 different OR’s mean many different things qualify as ostentatious as lone as 1 or more of the following applies.

  seeking to attract attention
  seeking to attract admirarion
  seeking to attract envy
  attracting attention
  attracting admirarion
  attracting envy

The “often by” means it isn’t required for the definition to apply. Gothic cathedrals therefore qualify as they where designed to attract attention and admirarion and in most instances also achieved that goal even without a garish color scheme or tons of gold inlays etc.

> You even use the word "tacky", so clearly you aren't blind to at least the general idea of bad taste, even if we may be in disagreement about what constitutes bad taste.

FALSE. Vulgarity or crudeness isn’t part of the definition. I used “tacky” as a specific qualifier to distinguish vulgar from non vulgar examples of ostentation.

A peacock display generally described as ostentatious but nobody calls it tacky. https://www.audubon.org/news/a-literal-bird “Peacocks have some of the most ostentatious (and famous) feathers in the animal kingdom.” People using those feathers can definitely get tacky. But essentially tacky is ostentatious displays done poorly.

> The 2 different OR’s mean many different things qualify as ostentatious as lone as 1 or more of the following applies. [...] The “often by” means it isn’t required for the definition to apply.

Read in such a decontextualized, selective, and uncharitable manner renders the definition worthless. For example, is any attempt to attract attention ostentatious? Any attempt to attract envy? Admiration? I think not. So your reading is inadequate and defective. Something is presupposed by the definition for it to make sense, which is to be expected, as dictionaries are not compendia of mathematical formulas written in some rigorously formalized language defined in the preface.

The first part of the definition does indeed use the word "often" which appears to soften the relation between gaudiness and the rest, but it still establishes a generality, i.e., that gaudiness is a reliable mark of ostentation. Otherwise, what's the point of including that in the definition if it isn't somehow characteristic of ostentation? It would not be a matter of definition, just an incidental thing that may or may not be true.

Note also the second part of the definition: "overly elaborate or conspicuous". Something that is overly elaborate conspicuous has transgressed some norm, has it not? Otherwise, how can you judge that something is "overly" elaborate? And if beauty is what satisfies a norm, then departure from the norm, either by defect or excess, is a deviation and so the thing in question has moved away from beauty toward ugliness. The technique used may be refined, but the composition can still be ugly. (And note that I, too, used an "or" in "The key to ostentation is vulgarity or crudeness, ugliness". I didn't think it needed to be spelled out.)

Now take the Oxford Dictionary's definition, which is arguably a bit sharper in this regard:

  pretentious and vulgar display, especially of wealth and luxury, intended to impress or attract notice.

This definition leaves no doubt about the relation of vulgarity and pretentiousness to ostentation.

> A peacock display generally described as ostentatious but nobody calls it tacky.

Yes, people can use it that way, but this is an analogical use of the term. But the use of "ostentatious" in your original post was not this meaning, at least not according to a normal reading given the sum of all context clues. In fact, you wrote:

> I think Klimt is the only artist to successfully incorporate gold into his paintings without making them look obnoxious or ostentatious.

Wait a minute! Klimt's use of gold could be heavy by any standard. Most iconography, and most art that uses gold, in fact, doesn't lean into gold as much as Klimt does in some of his works. So something doesn't add up here. The only way you could make that claim is if the aim of Klimt's work, its form, makes legitimate such heavy use of gold. Furthermore, you say "obnoxious and ostentatious". Not pejorative? Could have fooled me! Is some defect, something vulgar OR crude OR ugly being suggested? How could something beautiful be obnoxious?

Picking one of multiple definitions from the OED when others exist is disingenuous.

ostentatious, adj.

Of actions, events, qualities, etc.: performed, exercised, or displayed in a manner calculated to attract attention or admiration

One of the examples is: “He gave an ostentatious yawn.”

So, sure often it’s used in the context you’re referring to but that doesn’t mean any use is limited to that context.

> For example, is any attempt to attract attention ostentatious?

Attempts to grab attention are inherently a transgression, remember kids in school being admonished for attention grabbing behavior? Raising your hand is an attempt at communication, raising your hand and waving wildly it is an attempt to “gab attention” there’s a difference.

Advertising blockers removes even pure text advertising because grabbing attention is inherently a transgression. But, we don’t classify all transgressions as problematic. Someone breaking the speed limit taking someone to the hospital in a major medical emergency is generally accepted though not if they’re doing 190 MPH. A wedding dress has more leeway to be ostentatious than most outfits, but the line of acceptable has moved not been removed.