This book constitutes the proceedings of the 21st International Conference on Web Engineering, ICWE 2021, which was supposed to be held in Biarritz, France, in May 2021. The papers were organized in topical sections named: Semantic Web; Web modeling and engineering; Web big data and data analytics; Web user interfaces;

Semantic Web.- Interface to Query and Visualise Definitions from a Knowledge Base.- CARDINAL: Contextualized Adaptive Research Data description INterface Applying Linked data.- Publishing base registries as Linked Data Event Streams.- OntoSpect: IoT Ontology Inspection by Concept Extraction and Natural Language Generation.- A File-based Linked Data Fragments Approach To Prefix Search.- Social Web.- Assessing the Quality of Online Reviews using Formal Argumentation Theory.- Web User Interface as a Message: Power Law for Fraud Detection in Crowdsourced Labeling.- Conversation Graphs in Online Social Media.- Web Modeling and Engineering.- WTA: Towards a Web-based Testbed Architecture.- Towards Large-scale Empirical Assessment of Web APIs Evolution.- Stability Metrics for Continuous Integration of Service-Oriented Systems.- Web Big Data and Data Analytics.- Hybrid Collaborative Filtering for Rating Conversion in Recommender Systems.- Sentence Dependent-Aware Network for Aspect-Category Sentiment Analysis.- Lawless and Owen Conlan A Probabilistic Approach to Personalize Type-based Facet Ranking for POI Suggestion.- Web Mining and Knowledge Extraction.- Web Table Classification based on Visual Features.- Automated Essay Scoring via Example-Based Learning.- Conversation and Recommendation: Knowledge-Enhanced Personalized Dialog System.- MPIA: Multiple Preferences with Item Attributes for Graph Convolutional Collaborative Filtering.- Better Call the Plumber: Orchestrating Dynamic Information Extraction Pipelines.- Preprocessing Techniques for End-to-End Trainable RNN-based Conversational System.- Effective Seed-Guided Topic Labeling for Dataless Hierarchical Short Text Classification.- PrivaSeer: A Privacy Policy Search Engine.- Web of Things.- Knowledge-Driven Architecture Composition: Assisting the System Integrator to Reuse Integration Knowledge.- A-MaGe: Atomic Mashup Generator for the Web of Things.- WebAssembly Modules as Lightweight Containers for Liquid IoT Applications.- Leveraging Web of Things W3C recommendations for knowledge graphs generation.- A Standalone WebAssembly Development Environment for the Internet of Things.- Web Programming.- Full Stack is Not What It Used to Be.- An Improving Approach for DOM-based Web Test Suite Repair.- Communicating Web Vessels: Improving the Responsiveness of Mobile Web Apps with Adaptive Redistribution.- Snapshot-based Migration of ES6 JavaScript.- Web User Interfaces.- Automated Repair of Layout Bugs in Web Pages with Linear Programming.- A Model-based Chatbot Generation Approach to Converse with Open Data Sources.- Open Data Accessibility based on Voice Commands.- PWA vs the Others: A Comparative Study on the UI Energy-efficiency of Progressive Web Apps.- Ph.D. Symposium.- Static Analysis of Large-Scale JavaScript Front End.- Applying Predictive Analytics on Research Information to enhance Funding Discovery and strengthen Collaboration in Project Proposals.- A Web-based Co-creation and User Engagement Method and Platform.- Posters and Demonstrations.- Effectiveness comparison of email addresses recovery from Gravatars.- A Web Tool for XQuery Debugging.- Managing Versioned Web Resources in the File System.- Visualizing Web Users' Attention to Text With Selection Heatmaps.- City-Stories: Combining Entity Linking, Multimedia Retrieval, and Crowdsourcing to Make Historical Data Accessible.- SMOTE: A Tool to Proactively Manage Situations in WoT Environments.- Voice-based Virtual Assistants for User Interaction Modeling.- Tutorials.- Similarity Search, Recommendation and Explainability over Graphs for different domains: Social Media, News Industry, and Health.- High-level Interaction Design with Discourse Models for Automated Web GUI Generation.- p. 495 George Panagopoulos and Fragkiskos Malliaros Influence Learning and Maximization.