Yes, easily, with an ordinary keyboard, and chiefly without extra software ! Vim (on Linux) is an effective text editor for html encoding ; on Mac, it runs in "Terminal".
The setting of its configuration file "HTML.vim" allows, with a simple routine for typing, to automatically code the special characters to UTF-8. The HTML.vim file is included in the hidden directory .vim, located at the root of your "home", and you can download this tiny directory here (1,3 ko). Its inspection allows to quickly understand the routine, and modify it at your taste (for those who still prefer iso8859 coding, you can download the corresponding micro-directory here).
Typing any special character begins by ":" then comes its letter, and finally its description.

For vietnamese

Description :
u is cup, c is crocheting (for the o and the u), h is hat (circumflex = dấu mũ), a is acute (dấu sắc), g is grave (dấu huyền), t is tilde (dấu ngã), q (like "question") is interrogative (dấu hỏi), p (like "point") is fall tone (dấu nặng).

Examples :
:au gives ă (a-cup), :oc gives ơ (o-crocheting) et :uc gives ư (u-crocheting)
:Aa gives Á, :auq (or :aqu) gives ẳ, :eht (or :eth) gives ễ, :Ocp (or :Opc) gives Ợ, :ucg gives ừ
to obtain sắc, one must type s:auac        to obtain ngã, one must type ng:at        to obtain nặng, one must type n:aupng

For esperanto

it is simpler, for the number of special characters is much lower : ĉ ĝ ĥ ĵ ŝ ŭ   Ĉ Ĝ Ĥ Ĵ Ŝ Ŭ

Description :
h means hat, and u means cup

Examples :
:ch gives ĉ, et :Uu gives Ŭ
to obtain ĉirkaŭ, one must type :chirka:uu         to obtain paĝo, one must type pa:gho

Special characters existing on the french keyboard are also automatically encoded in UTF-8 (for the same price !:-) ; for example, if I type è, Vim encodes è

The produced .html page can at once be copy-paste in your everyday text editor or your e-mail client, or uploaded.