Computers, mobiles and tablets can send fax's with software, its just like sending an email, it was added to rfc2086 15 years ago but it never took off and as far as I know no major browsers support href="fax:"
they do however support href="tel:"
that can be used just the same to send a fax using software.
2.3 "fax" URL scheme
The URL syntax is formally described as follows (the definition
reuses nonterminals from the above definition). For the basis of this
syntax, see [RFC2303] and [RFC2304].
fax-url = fax-scheme ":" fax-subscriber
fax-scheme = "fax"
fax-subscriber = fax-global-phone / fax-local-phone
fax-global-phone = "+" base-phone-number [isdn-subaddress]
[t33-subaddress] [post-dial]
*(area-specifier / service-provider /
future-extension)
fax-local-phone = 1*(phonedigit / dtmf-digit /
pause-character) [isdn-subaddress]
[t33-subaddress] [post-dial]
area-specifier
*(area-specifier / service-provider /
future-extension)
t33-subaddress = ";tsub=" 1*phonedigit
The fax: URL is very similar to the tel: URL. The main difference
is that in addition to ISDN subaddresses, telefaxes also have an
another type of subaddress, see section 2.5.8.