Just a few quick words on converting to WebM:
Never use online converters, who knows what they do with the conversion.
For converting on your own, the foremost software right now are the following.
For converting to VP8/Vorbis you can use WebMConverter:
An easy to use interface, it can also convert to VP9/OPUS, but then it has issues with filesize accuracy, but that's really only a problem when converting for posting on chans, where you have a filesize limit.
For converting to VP9/Opus you should use Staxrip:
Not as user-friendly as webmconverter, it has a lot more room for user error, but the end results are great. It is capable of converting to several different codecs and filetypes, but only VP9 WebMs.
FFMPEG, the software that both the previous ones use to actually encode the video:
Being command-line interface the word "user-friendly" doesn't even enter into it. Capable of the highest quality video output, but you pretty much have to be a wizard to achieve it.