Extract the dictionary (the dictionary has an adm or adu extension) to HTML Validator's dictionary folder. This is usually one of the following folders, but it depends on the version and edition that you're using and whether you're using a 32-bit or 64-bit OS (the vast majority of OS's are 64-bit these days):
C:\Program Files\HTMLValidator240\Dictionary (2024/v24 with 64-bit OS)
C:\Program Files\HTMLValidator230\Dictionary (2023/v23 with 64-bit OS)
C:\Program Files\HTMLValidator210\Dictionary (2021/v21 with 64-bit OS)
Open (run) CSS HTML Validator.
Choose Options > Editor Options (Ctrl+F3), go to the Editor > Spell Checking page, and then press the More Spelling Options button.
Choose the Language tab. The dictionary that you added should now be listed in the Additional dictionaries box. Select all the dictionaries that you want to use and unselect those that you don't want to use. For example, if you want to use the English (British) dictionary, then select that dictionary and unselect any other English dictionaries.
Technical -
Contains words from Mathematics, Statistics, Computer Science, Accounting and other Scientific areas, thus may be applicable to more than just English users.
Create Your Own User Dictionaries
This dictionary utility allows editing, creation, import, and export of custom dictionary data.