20% of what? Unrealized gains. Do they get tax deductions or credits for unrealized losses? What the market giveth, the market also taketh away.
The article linked in the OP is a bit unclear. It's my understanding that there are two salient proposals being made. One is to impose a minimum 20% tax on the income of billionaires - which would probably involve denying them the foreign tax credits that have been available almost since the income tax was enacted - and the second is to tax billionaires on the unrealized gains in their assets.
The second would affect the first in practice, but logically they are two distinct proposals.
With respect to the proposal to tax unrealized gains, it would probably end up being something like the mark-to-market regime that a taxpayer who owns marketable shares of stock in a PFIC can elect to apply to those shares under IRC 1296. Basically, market gains that have accrued during the year are reported as taxable income, and market losses that have accrued during the year are allowed, but only to the extent that the taxpayer has previously reported unrealized gains with respect to the asset that is now giving rise to the loss.
So, for example, assume an affected individual buys a stock for $100 on January 1 of Year 1. If the market value of that stock is $150 at the end of Year 1, then the individual would report $50 of unrealized gain income for Year 1 and would pay tax on it (most likely at the ordinary rates). That should result in the individual having an adjusted cost basis in the stock of $150 at the end of Year 1/beginning of Year 2. If, at the end of Year 2, the stock's value has dropped to $130, then the individual would probably be allowed to claim a loss of $20 for Year 2 since the individual's cumulative net inclusions for all prior years are $50. This loss would most likely (under current principles) be treated as an ordinary loss, not a capital loss.
If, at the end of Year 3 the stock had further declined in value to $90, then the individual would be allowed to claim a further loss of $30, because that is equal to the amount of prior-year unrealized gains the individual had previously reported as income. The remaining $10 of loss would not be allowed.
If, however, the individual were to have actually sold the stock for $90 during Year 3, then the individual would be allowed to claim the loss of $30 as recapture of previously reported unrealized gains, and - depending on the policy choices actually made - would then have a further $10 loss which would most likely be characterized as a capital loss and therefore subject to the $3,000 limitation on capital losses that exceed capital gains for the year.
Basically, the system would be unfair. It would tax an affected individual on all of the gains in his assets at ordinary rates, not at the lower capital gains tax rate, and would only allow actual capital losses to the extent those losses either offset other capital gains, or allow them to offset up to $3,000 of unrelated ordinary income.