I'd apply the term 'alt-left' to those on the left who are alienated from the leadership of the Democratic party and from the positions on various issues that leadership takes. Bernie Sanders and his supporters represent one variety of that tendency, the remaining campus Marxists and the 'occupy' and 'black lives matter' rioters another.
Hillary was the ultimate insider, very much the representative of the Democratic party establishment, the antithesis of the 'alt-left'. With her defeat, I expect the 'alt-left' to make a move to take over the party, much as the Corbyn-ites did with the Labour party in Britain.
As practical as any, since alt-right treated as a broad category (with better origins) should have also referenced "alienation from Republican party establishment", with plug-in details for any specific applicable group. What alt-left is supposed to be, is semantically amorphous and freer at this stage -- compared to the accumulated history that devours alt-right.
As a potential generic label, "alt-right" seemed hopelessly constricted and damned because of Richard Spencer coining it and having once had the website. Till the more heterogeneous lot of Trump supporters / voters came along and muddled up that broad paint brush. Even when going back to the super-negative usage of "alt-right" and narrowing it down as an active movement, there's the irregular skepticism of it being regarded as rude theater as much as a legitimate monster.
In context of that interpretation... The alt-right is a media bogeyman created by the online activity of blasphemous, slowly maturing 15 to 68 year olds behaving like frat-boys. Kind of like the more unruly members of '60s and '70s counterculture poking everything sacred in the eye ("Just give me a superficial ideological excuse to do something crazy!"). Only this time the establishment that's having its totems urinated on (by these inverted-spectrum hipsters) is a grown-up descendant of that earlier era of rebel reformers. As well as the conservative mainstream being part of the target.[*]
Similarly, the embryo stages of a "far-out" movement for the alt-left (AKA metamodernism and other tentative attempts to fill in the placeholder) are equally open to the question: "Is this for real or just more online antics (either facetious, bizarre or extreme)?"
-->Left Reality -->Left Reality Groups
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[*] Andrew Marantz: The alt-right has no consistent ideology; it is a label, like “snob” or “hipster,” that is often disavowed by people who exemplify it. The term typically applies to conservatives and reactionaries who are active on the Internet and too anti-establishment to feel at home in the Republican Party. --Trolls for Trump
Benjamin Wallace-Wells: [...] One way to understand the alt-right is not as a movement but as a collective experiment in identity, in the same way that many people use anonymity on the Internet to test more extreme versions of themselves. Moldbug, when he stepped out from behind his pseudonym, turned out to be a Silicon Valley computer programmer who had started as a commenter in the factional circles of libertarian message boards. CisWhiteMaelstrom, who convened the pro-Trump hordes that swallowed the politics sections of Reddit, turned out to be a law student in his early twenties who was looking forward to a job in which he could make the most money possible.
[...] as an ideology, it can be hard to take the alt-right seriously. When [Richard] Spencer named the movement, he was the managing editor of *Taki’s Magazine* [...] Its own propagandists often say they are joking. The right-wing provocateur Milo Yiannopoulos, of Breitbart, himself a leading fellow-traveller, claimed that some “young rebels” are drawn to the alt-right not for deeply political reasons but “because it promises fun, transgression, and a challenge to social norms.” The alt-right exists mostly online, and so it is shrouded in pseudonyms.
The strains that run through the alt-right [...] are in their essence not matters of substance but of style. They share with the Trump movement a haughty success theatre that complicates their populism: the alt-right’s defense of the white working class [...] is not an instance of self-preservation but of “noblesse oblige.” The two also share the instinct for provocation. “If you spend 75 years building a pseudo-religion around anything—an ethnic group, a plaster saint, sexual chastity or the Flying Spaghetti Monster—don’t be surprised when clever 19-year-olds discover that insulting it is now the funniest fucking thing in the world,” the blogger Mencius Moldbug wrote to Yiannopoulos.
The alt-right often seems to be testing the strength of the speech taboos that revolve around conventional politics—of what can be said, and how directly. --Is the Alt-Right for Real?
Chava Gourarie: [...] it’s unclear that the alt-right “movement” even qualifies as one. Because of the nebulous nature of anonymous online communities, nobody’s entirely sure who the alt-righters are and what motivates them. It’s also unclear which among them are true believers and which are smart-ass troublemakers trying to ruffle feathers. [...] The nature of such communities is that it is almost impossible to know whether they are actually ideological, or making noise for the thrill of the response. [...] Shapiro says the vast majority of alt-righters are motivated less by ideology than by an itch to instigate. They like Trump because of his ability to get away with the kind of speech they revel in. [...] On the internet, whoever is loudest or best at animating the Twitter hordes wins for the moment [...] The media has to respond to the alt-right’s antics, but by responding we’re playing directly into their hands. [...] if the primary purpose of the alt-right is to provoke, then attention is their life force, and media attention their fuel. [...] “Getting a journalist to repeat a racist meme is part of the game,” says Whitney Phillips, the author a book about 4chan and troll subcultures called *This Is Why We Can’t Have Nice Things*. “It’s part of the goal.” --How he ‘alt-right’ checkmated the media