I find that the font is too small for some users to read. Also the width of the legend takes up to much room when embedded in a webpage.
Which app are you using? If you are using a configurable app, it may be possible to modify the legend and font depending on the font type and app using custom CSS:
Modify configurable app appearance using custom css | ArcGIS Blog
If you are using Web AppBuilder for ArcGIS, this may be possible with the developers edition, but I've tagged in the space for someone more knowledgable than me to chime in.
It looks like you are using WAB and the launchpad theme. You can make css changes like that if you are using the Developer edition. But if you are using AGOL then then only thing you can do is configure the app not make edits like that.
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