How to fix a grayscale border when downloading html elements with html2 canvas

I am building a sudoku puzzle generator + sudoku downloader. There is no problem with the sudoku itself, but with the downloaded image of it.

So, I'm using html2 canvas, which allows me to download html elements. I did this with the sudoku. The big problem is that when I download that html element, I see some grayscale around the borders, as you can see in this example:

And in the website you can't see that grayscale border:

This probably does not have to do anything with css, because there is no problem in the styling itself, but in the way the html was downloaded.

I tried using different image formats, like png, jpg and heic, the results were all the same.

If someone knows anything that could solve my problem, please let me know.

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