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:
C:\Program Files\HTMLValidator200\Dictionary (2020/v20 with 64-bit OS)
C:\Program Files\HTMLValidator190\Dictionary (2019/v19 with 64-bit OS)
C:\Program Files\HTMLValidator180\Dictionary (2018/v18 with 32-bit OS)
C:\Program Files (x86)\HTMLValidator180\Dictionary (2018/v18 with 64-bit OS)
C:\Program Files\HTMLValidator170\Dictionary (v17 with 32-bit OS)
C:\Program Files (x86)\HTMLValidator170\Dictionary (v17 with 64-bit OS)
C:\Program Files\HTMLValidatorLite160\Dictionary (v16 lite edition with 32-bit OS)
C:\Program Files (x86)\HTMLValidatorLite160\Dictionary (v16 lite edition 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.