Including constructs such as “⊕” in an html file produces a variety of special symbols such as “⊕” in this case. Utf-8 is a more compact form that may be easier to use depending on the tools you use to make web pages. Utf-8 is one of the native languages of TextEdit which comes with Mac’s OS X. Here is a series of code pages; you can see how your browser displays these characters. The character is shaded unless its code is specified in Character entity references in HTML 4. too. This page presents much of the same information in another form. Perhaps these or these are the most official and complete. I don’t know any browsers that come near to implementing a large fraction of these however.

Internet Explorer 5.1 for Mac OS X, and Netscape 6.2.1 for Mac OS X agree on just about all of these official codes. “ϖ”, “℘”, “ℜ” are the only exceptions that I have noticed.

There are many other codes for which the two browsers agree. They come in clumps and someone clearly hopes they will become standard.

There are as well many scattered fragments where the two do not agree. Usually the two will agree or one will display the question mark.

Main Official Clumps
Greek, Cyrillic, Punctuation, Exotic letters, Arrows and aleph, Math, etc., etc., Check out this indigenous Chinese Yi syllabary.
Unofficial common clumps; IE & Netscape mostly agree
Misc Tech ⑭ ⑛ ⒲, Lines and corners, Geometric forms, Cards, Music, Units
And even Chinese, Georgian, Hebrew, Japanese, Korean,
See the deprecated Symbol characters too. Netscape 6 is somewhat deficient in these.

Here are hexadecimal character entity references which access the same glyphs but follow the hexadecimal grain of the allocation of code blocks. If you type or paste an exotic character into the box, it will take you to the code page. Note that the end of the URLs to these pages are hexadecimal code block numbers which you can generate by editing the URL in the address bar. math, Greek, Cyrillic, Japanese, Chinese,

A∧B̅∧C̅
I hope you see in the line above bars over B and C. I edited this html file with Apple’s TextEdit app and got the bars over a letter by putting the cursor just after the letter and pasting a clip-board content I had cut (using Chrome or Safari) from the 0 row of this page. Alternatively “ẋ” produces “ẋ”. Starting with “x̀” and counting up in decimal we have these combiners:
ò: à é
ô: ô ñ ē e̅ ĕ ė ü ẻ Å e̋
č: č e̍ e̎ ȅ e̐ ȇ e̒ e̓ e̔ e̕
e̖: e̖ e̗ e̘ e̙ e̚ e̛ e̜ e̝ e̞ e̟
e̠: e̠ e̡ e̢ ẹ e̤ e̥ e̦ ç ę e̩
e̪: e̪ e̫ e̬ ḙ e̮ e̯ ḛ e̱ e̲ e̳

Internet Explorer once thought that modifiers preceded the modified character. But this says “A combining character sequence is a base character followed by any number of combining characters.”.

Discrepancies

On my 2016 Mac, OS X 10.11: Unicode 2263 (≣) displays as four horizontal bars in TextEdit for any of the 20 randomly selected fonts that I tried. It shows as 3 horizontal bars in the ‘Characters’ tool invoked in TextEdit or Mail: Menu > Edit > Emoji & Symbols where the error shows both under “Unicode” and “Math Symbols”. This shows 4 bars.

bad Phi (φ)

With these links to 100 files each with 100 such constructs; The CGI logic to produce these character tables.
pronunciation (add word or letter at end.)