I have visited user3700643's page and see that there are still problems--- this when I use Firefox's Responsive Design View at 320×487. My solution is doofus I know, as it requires some duplication, but I have used it with not only MathJax (just now) but in the past for tables and other things--- all in a Twitter Bootstrap grid environment, but I'm pretty sure that that doesn't matter.
I have been using the CSS3 @media Rule. Here is my particular version:
@media(max-width:768px){
.wide {display:none;}
.mobilefont {font-size:0.8em;}
}
@media(min-width:768px){
.tight {display:none;}
}
I just threw the mobilefont bit in just to further inspire you. I do sometimes use it. So, what I do is wrap an equation--- I'm using Latex with MathJax--- with a div and give the div the class "wide" if the equation is formatted for a wide screen and "tight" if it's formatted for a small screen. You can see how the max-width/min-width values in the CSS3 effect the disappearance of divs that have those class names according to whether the screen width is greater than or less than 768px, which is the popular breakpoint (but others can be defined by you).
That is, I duplicate my big-screen \begin{align}\end{align} block. I paste the copy in right under the original and wrap each with divs and give the two divs those two class names. I then edit the one with the "tight" class name to make it work in the Responsive Design View with a small screen size.
I have also done this outside of the MathJax context, with plain tables. I have, for example, used <td class="wide"> in every <td> of a given column of a table to make that column disappear on small-screen devices.
On your page where you have "L'integrale di partenza sarà allora la somma degli integrali sui due domini", inside your "tight" div version you could, if you were using a \begin{align}\end{align} block (but I see you're actually using \begin{eqnarray*} which I haven't tested with this scheme), simply put an ampersand in front of that double integral on the left, and then new-line characters \\ after that integral to break to a new line. And put ampersands in front of each equal sign on every line. That way the lines that start with equals signs will be moved far to the left, aligned under the expression for the integral instead of to the right of it.
That would not be too confusing, I don't think. It's only slightly unorthodox. But see my comment to Peter Krautzberger's answer... I have found that his script doesn't work when I use the ampersand. But the point is that it isn't needed at all with my CSS3 approach since I simply specify all of the re-formatting myself.