Due to considerable progress by the Syrian government against the foreign sponsored, jihadist insurgency new allegations have to be found to justify additional foreign intervention.
These now come in the form of alleged use of chemical weapons by the Syrian government:
"Shrunken pupils, foaming at the mouth and other signs indicate, in our view, that lethal chemical weapons were used."
The U.S. government does not support these unsubstantiated allegations.
While "foaming at the mouth" can be a sign of chemical weapon exposure other possible causes for apparent "foaming at the mouth" do exist.

A man "foaming at the mouth"
One therefore should be cautious about the "proof" under the Telegraph's headline Syria doctor's Facebook video proof that Assad used chemicals in Aleppo:
Syria's regime dropped chemical weapons from a plane on a district of Aleppo earlier this month, killing two infants and a woman, experts said after a doctor in Aleppo posted a video of apparent victims to his Facebook page.
…
Niazi Habash, a British-trained doctor who treated the Aleppo victims, said they showed symptoms of exposure to chemicals, including breathing difficulties, foaming at the mouth and pinprick pupils.
The short video posted at the Telegraph site shows three persons laying on hospital stretchers. All three persons have some white foam around their mouths. None of them shows any acute breathing problem. All three seem rather relaxed. Here are three screenshots from the doctor's video showing each of the three "victims". You may judge yourself what kind of "chemical weapon" might have produced their "foaming at the mouth" symptom.

A man "foaming at the mouth"

Another man "foaming at the mouth"

Another person chewing on a tube and with heavy "foaming at the mouth"
This "foaming at the mouth" video proof of chemical weapons usage is fake. That is not to say that the above people are not under chemical attack:
[M]any of these shaving creams feature more harmful ingredients than beneficial ones. Here's what to keep an eye out for.
Propolene glycol is a humectant like glycerin, but unlike glycerin, it's more frequently found in antifreeze and brake fluid. Triethanolamine, better known as TEA, is an emulsifying agent, meaning it helps keep the oil and water from separating. It's also a very controversial ingredient in the cosmetic industry because not only is it a skin irritant, but many formulas containing TEA are found to be contaminated with nitrosamines, which are linked to cancer. Sodium lauryl sulphate (SLS) and sodium laureth sulphate (SLES) are two more controversial skincare ingredients. These make a great lather, but they also have a number of health implications. Lauryl mimics estrogen, which is especially problematic for women, and laureth often hosts a known carcinogen called dioxane.
Should we bomb Assad for this massive chemical shaving cream attack? Maybe not.
Faking videos of alleged bombing and allegedly wounded victims has been a standard practice of the Syrian insurgents.
This "foaming at the mouth" shaving cream video posted by a doctor who is on the side of the insurgency is just another example of such fakery.
But the Telegraph has an additional "expert" judging about the "chemical weapon" usage:
Hamish de Bretton-Gordon, a former British Army specialist and director of a biological consultancy, said the most likely explanation was the chemical sarin had been used.
"From what we've seen, and the descriptions of containers being dropped from the air, it certainly seems that the regime is using sarin in an unprepared state in these attacks," he said.
That a is somewhat funny "expert opinion" by a former tank regiment commander from a "biological consultancy" about "chemical weapons" as "foaming at the mouth" is NOT a standard symptom of sarin exposure. Yet, looking at fake videos with shaving foam, he claims to see sarin usage.
These fake videos and fake experts are not proof of anything but of warmongering by certain "western" media.