What WEBM to MP4 actually writes
Screen recordings and browser captures are often WEBM, and WhatsApp or an editor then refuses them.
The WEBM is re-encoded to MPEG-4 video and AAC in an MP4. The first run downloads the converter. Stay on the tab until it finishes.
Quality and the WEBM file you should keep
A screen recording that was already soft will stay soft. This does not add resolution.
A related step, if you still need one, is MP4 to WEBM.
Why WEBM to MP4 is its own job
WEBM to MP4 sits on this site because WEBM and MP4 are not interchangeable, even when they look similar in a file list. Screen recordings and browser captures are often WEBM, and WhatsApp or an editor then refuses them. Browsers like WEBM. WhatsApp, older phones, and many editors still want MP4. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4.
The WEBM is re-encoded to MPEG-4 video and AAC in an MP4. The first run downloads the converter. Stay on the tab until it finishes.
The WEBM file you already have
WEBM is the open video format browsers record and play. Screen captures are often WEBM. The encode here uses VP8 video around 1 Mbps and Vorbis audio, so size and sharpness will not match a high-bitrate MP4.
Keep the MP4 or the original capture if you might edit it.
What a MP4 result is for
The download has to be MP4, because that is the format the next step named. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4. If the other person can open only one video format, MP4 is that format.
A new encode cannot look cleaner than the MP4 you started with. A remux on a desktop is the lossless path when the streams are already compatible.
How to tell that WEBM to MP4 worked
WEBM to MP4 is worth using only when MP4 is what the next step actually named. Screen recordings and browser captures are often WEBM, and WhatsApp or an editor then refuses them. What you get from WEBM to MP4: The WEBM is re-encoded to MPEG-4 video and AAC in an MP4. The first run downloads the converter. Stay on the tab until it finishes. Before you trust it: A screen recording that was already soft will stay soft. This does not add resolution. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. A second pass rarely helps. If the first MP4 file is rejected, read the error from the program that rejected it. Another conversion of a bad result usually makes the file worse, not more compatible. When you are done, you should still have the WEBM original and one MP4 download that you have opened. If those two are the same kind of failure, this was the wrong tool and the page above is the place that says so. Video files hide the real problem inside the container. WEBM to MP4 re-encodes on your device. It does not silently copy every audio language and every subtitle the way a desktop remux can. If you needed those extra tracks, this download is the wrong result even when the picture plays. Convert a one-minute piece before a full movie, and stay on the tab while the converter downloads the first time.
Here is the decision for WEBM to MP4. You start with WEBM and you stop when a real program accepts MP4. It is a reasonable web playback file. Soft subtitles such as ASS or PGS, and extra audio languages, are awkward or dropped when a tool re-encodes. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
A screen recording that was already soft will stay soft. This does not add resolution.
A related step, if you still need one, is MP4 to WEBM.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.