This document uses charset=UTF-8 and in FrontPage page properties, the FP Language is set to Thai. 

So:

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta http-equiv="Content-Language" content="th">
 

ฟหกดึ้

This is some Thai stuff: กหืกยท่ See?

 

An experiment with entering data via the "&#" notation: 

When entering in HTML mode in Frontpage, you can type the &# notations and see them.  But after any editing in Normal mode and then returning to HTML mode, the chars are replaced (in HTML view) with the actual glyphs or boxes.

1. Two Khmer chars: hex ក ខ  - they showed up only after I added the shareware Code2000 font. So Khmer is not in MS Arial Unicode (even when the Font is set to Arial Unicode - see below Cyrillic and Greek)

2. Two Burmese chars: 1010 and 1020 (hex) = 4112 and 4128 decimal: တ ဠ (these still show in &#notation but do not show even when font is set to Arial Unicode.  So they are not in the MS Arial Unicode font)

3. A Thai char in the "&#" notation: ฐ - char 0E10 = &#3600 decimal - but now we see the Thai here and in HTML view.

4. N-tilde (241): ñ - again, we now see n-tilde in HTML mode even though &#241 was typed in HTML mode

5. Cyrillic hex 0470 = dec 1136:  Ѱ - it doesn't show when font is Times New Roman but it does now having set Arial Unicode as the font.  But the Thai above (under 3.)  is "Normal" and "Times New Roman".

 6. Greek 03E2 = 994:  Ϣ - same deal as the Cyrillic.

 7. IPA:   02A3 = 675: ʣ - same deal as Cyrillic.

Another page, done a bit differently.