Difference between overflow-wrap and word-break?

Quoting from source

• overflow-wrap: The overflow-wrap CSS property is used to specify whether or not the browser may break lines within words in order to prevent overflow when an otherwise unbreakable string is too long to fit in its containing box.

• word-wrap: The word-wrap property was renamed to overflow-wrap in CSS3.

• word-break: The word-break CSS property is used to specify how (or if) to break lines within words

So, you need word-break in combination with word-wrap, which is the right combination.

It helps to understand that at this point, word-break: break-word is really an alias for overflow-wrap: anywhere.

word-break: break-word is officially deprecated; see the CSS Text Module Level 3 Working Draft:

For compatibility with legacy content, the word-break property also supports a deprecated break-word keyword. When specified, this has the same effect as word-break: normal and overflow-wrap: anywhere, regardless of the actual value of the overflow-wrap property.

The thing to note here is that word-break: break-word is an alias for overflow-wrap: anywhere, NOT an alias for overflow-wrap: break-word.

(word-break: normal is just the default value for word-break, so you can ignore it unless you're setting a different value for word-break.)

How do overflow-wrap: anywhere and overflow-wrap: break-word differ?

The only difference in the documentation between the two is that overflow-wrap: anywhere DOES "consider soft wrap opportunities introduced by the word break" when it is "calculating min-content intrinsic sizes", while overflow-wrap: break-word does NOT.

I guess widths might be more accurate in some cases if it is considering them?

Here are the exact differences: (based on testing in Chrome v81, and confirming my observations by referencing the spec)

white-space

normal (default): collapses whitespace-chains and line-breaks; adds line-breaks where needed
nowrap: collapses whitespace-chains and line-breaks; doesn't add line-breaks
pre-line: collapses whitespace-chains; adds line-breaks where needed
pre-wrap: no collapsing; adds line-breaks where needed
break-spaces: same as pre-wrap, except with spaces able to trigger line-break-adding
pre: no collapsing; doesn't add line-breaks

Note: If the selected white-space value lists "doesn't add line-breaks", the line-break behavior of the following properties is unable to be applied (ie. ignored).

word-break

normal (default): breaks line at end of last word fitting within container [if one exists], else line left unbroken
break-word: breaks line at end of last word fitting within container [if one exists], else at end of container
break-all: breaks line at end of container [can split a word, even with nearby whitespace]

overflow-wrap (legacy name: word-wrap)

normal (default): breaks line at end of last word fitting within container [if one exists], else line left unbroken
break-word: breaks line at end of last word fitting within container [if one exists], else at end of container [if in non-flex container], else line left unbroken
anywhere: breaks line at end of last word fitting within container [if one exists], else at end of container [so same as word-break: break-word]

Note that for overflow-wrap: break-word (as for any combination that leaves lines too long for the container), the unbroken line can cause a flex container to expand beyond the flex ratio specified (forcing other flex containers to shrink to account for the too-long content).

overflow-wrap emphasizes how to handle overflow, word-break emphasizes how to break words.