There are competing definitions of regressive taxes out there:
A regressive tax is a tax imposed in such a manner that the tax rate decreases as the amount subject to taxation increases.
In a textbook I use (Case, Fair, and Oster's Principles of Economics):
A regressive tax is a tax whose burden, expressed as a percentage of income, decreases as income increases.
By the Wikipedia definition, a sales tax would technically be a proportional tax as the rate is fixed regardless of how the base (consumption) changes.
However, by the textbook definition, and assuming that households with lower income consume a greater percentage of that income than households with higher income (i.e. have lower savings rates), a sales tax with a fixed rate would be regressive.
You might be inclined to suggest that the textbook was defining it specifically for income taxes where the Wikipedia article is defining it generally. Actually, though, the authors of the text make it clear they mean their definition to be general. They say that they define it in terms of income because "all taxes are ultimately paid out of income." I think that statement is a bit imprecise, but whatever. They go on to show an example that demonstrates that sales taxes are regressive (conditional on lower-income households saving less).
I agree with them on an intuitive level that sales taxes are regressive. But then why has Wikipedia decided on a definition that is independent of income, with one consequence being that sales taxes are proportional?