The fastest way to view an MD file online is to open the page and drag the file onto it: the Markdown renders in about three seconds with no install, no sign-up and no upload step.
Open the viewer — drag in your .md file →Installing an editor to glance at one file is the slow route; pasting into an online editor loses images and relative links. This viewer skips both: drop the .md file (or a README.md) on the home page and it renders instantly — tables, code blocks, checkboxes, footnotes. Multiple files at once are fine: open tabs are independent, and each stays in your browser. If you only need to read, not write, this is the shortest path from file to answer.
.md / README.md file onto the drop zone (or click to browse).