Curiously, the article mentions Icelandic for æ, but not for ð or þ, which are still on regular use in modern Icelandic, pretty much in the same Old English meaning of dental fricatives (ð is voiced, þ is voiceless).
/ðæt/
In my dialect, the th in with is unvoiced, though it looks like it is voiced in others:
/wɪð/ /wɪθ/
It also has further explanation of th is sometimes voiced and sometimes unvoiced depending on context.
One thing to note is that some English speakers pronounce /ð/ and /θ/ as allophones, likely due to their identical orthography. At the beginning and middle of words, it becomes voiced /θɪn/ and /θɪŋ/ (thin and thing) becoming /ðɪn/ and /ðɪŋ/, respectively. At the end of words, it remains unvoiced.
https://en.wikipedia.org/wiki/Voiceless_dental_fricative
th-fronting always catches my attention as well:
However the æ in Danish and Norwegian is like the OE one described in the article, between a and e, which is written ä in Swedish. Icelandic does not have that sound, and uses æ in the Latin way, representing the ai diphthong (as in English "fine").
The more common vowels also have fairly different pronunciations in the four languages.
Or rather, it's a relatively modern language.
1000 years ago there would have been forms of Germanic, Scandinavian and French being spoken - and 'English' developed as a mangle of those.
The proper French people I know do not consider English to even be a proper language. :_