The word seems to have changed in meaning over the past few years, especially the last year. My son is retweeting mockery of the alt-right, there are arguments at neoneocon whether the alt-right is racist, and the alt-right is being held responsible for the rise of Trump. This seems a considerable narrowing meaning for the term. I have regarded alt-anything as a catchall term, describing disparate groups not big enough to get their own names but differing from the "anything" substantially enough that they could not be counted on to go along more than 50% of the time. Alt-religion included various pagans, satanists, Zoroastrians, and new-agers, but also combos such as JewBu's, or modern expressions such as intense Trekkies. Greens were not really alt-left, because there were enough of them and closely enough defined that they constituted their own group. Intense greens such a Gaians might have a foot in both worlds, but the alt-left would also include Trotskyites, communitarians, and anarchists. Preppers could be alt-right, alt-left, or just Scouts who took their childhood lessons to heart. UU's might have fair overlap with folks at alt-religion, but they have an institutional solidity that gives them plenty of folks to interact with that they already know. The alts were a function of the internet, where people who had interests or opinions that were offbeat enough that they could only find a few others in a city of 100,000 could now have an entire online community.
As a postliberal, I considered and rejected the idea that I was a part of either the alt-right or alt-left. But I didn't think it an insane question. I would have called libertarians alt-right, because there are enough of them that they are a thing on their own. But I might call Randians alt-right, or SSPX Catholics, nationalists of many stripes, monarchists, or separatist fundamentalist Christians. Anti-immigrant groups, both the anti-illegals and anti-alls, are more common on rightist websites, even though they are more evenly divided among the voters. (Factor in black voters and you start to see it.) So those likely qualify as alt-right. Single-issue crusaders such as prolife or DOMA demonstrators and tax protestors are sort of half in. Additionally, I would include a broader, less-intense group of homeschoolers, Burkeans, Mormons, Buchananites, and conservative Jews; significantly for this discussion, I would include an enormous number of writers who hearken back* to writers and thinkers of previous eras - classicists, medievalists, theologians, philosophers. Thomas Sowell, John Derbyshire, Theodore Dalrymple - even Tom Stoppard and Tom Wolfe. I have called those alt-right in my mind, because they are clearly conservative, but just as clearly not in complete accord with modern American conservatives.
White supremacists are their own thing, seeming to be more Republican than Unaligned than Democrat 2:2:1 (numbers dated, from 2000 election). Stormfront is mostly unaligned, more Democrat than Republican, but uses conservative and American military symbolism almost exclusively. I don't know what to make of that. If you look specifically for alt-right sites you find a lot of those guys, but one is struck by the fact that everyone is arguing with them, and everyone is resorting to all-caps and Hitler/Stalin almost immediately.
So in my mind, neoneocon is alt-right, though she is currently distancing herself from the current usage; Grimbeorn and James and hell, most of my sidebar fits the bill. I now get it that this meaning is gone forever. Alt-right will mean Trumpsters, anti-immigrant, and high-decibel anti-PC from this day forth, and even forevermore.
*I use this phrase in full knowledge and conscious defiance of the convention that calls this a variant. It is now so common a phrase that it should soon be rejected as a cliche. Which is a perfect illustration of prescriptivists: they dig in their heels for so long long that their very argument becomes archaic. (And yet, I used the double-space after the period and the Oxford comma throughout. Personal idiosyncrasy.)