H5Web is a web-based HDF5 file viewer with NeXus support. It allows for the browsing and inspecting of the hierarchical structure of HDF5 files, as well as visualising their datasets as basic plots (1D, 2D). With H5Web, our intent is to provide synchrotron users with an easy-to-use application and to make open-source components available for other similar web applications. H5Web is built with React, a front-end web development library. It supports the exploration of HDF5 files, requested from a separate back-end for modularity (e.g. HSDS, Jupyter, etc.), and the visualisation of datasets using performant WebGL-based visualisations.