Found the following HTML validation problems:
callback.html:207:6: Warning: missing </span> before <p>
callback.html:207:94: Warning: inserting implicit <span>
callback.html:210:6: Warning: missing </span> before <p>
callback.html:210:93: Warning: inserting implicit <span>
callback.html:324:6: Warning: missing </span> before <p>
callback.html:324:95: Warning: inserting implicit <span>
callback.html:452:6: Warning: missing </span> before <p>
callback.html:452:95: Warning: inserting implicit <span>
callback.html:577:6: Warning: missing </span> before <p>
callback.html:577:95: Warning: inserting implicit <span>
callback.html:580:6: Warning: missing </span> before <p>
callback.html:580:94: Warning: inserting implicit <span>
callback.html:217:1: Warning: <a> anchor "method-new" already defined
callback.html:231:1: Warning: <a> anchor "method-receive" already defined
callback.html:245:1: Warning: <a> anchor "method-continue" already defined
callback.html:259:1: Warning: <a> anchor "method-clone" already defined
callback.html:331:1: Warning: <a> anchor "method-new" already defined
callback.html:345:1: Warning: <a> anchor "method-receive" already defined
callback.html:359:1: Warning: <a> anchor "method-result" already defined
callback.html:373:1: Warning: <a> anchor "method-finally" already defined
callback.html:387:1: Warning: <a> anchor "method-clone" already defined
callback.html:459:1: Warning: <a> anchor "method-new" already defined
callback.html:473:1: Warning: <a> anchor "method-receive" already defined
callback.html:487:1: Warning: <a> anchor "method-result" already defined
callback.html:501:1: Warning: <a> anchor "method-finally" already defined
callback.html:515:1: Warning: <a> anchor "method-clone" already defined
callback.html:587:1: Warning: <a> anchor "method-new" already defined
callback.html:601:1: Warning: <a> anchor "method-receive" already defined
callback.html:615:1: Warning: <a> anchor "method-result" already defined
callback.html:629:1: Warning: <a> anchor "method-clone" already defined
callback.html:207:6: Warning: trimming empty <span>
callback.html:210:6: Warning: trimming empty <span>
callback.html:324:6: Warning: trimming empty <span>
callback.html:452:6: Warning: trimming empty <span>
callback.html:577:6: Warning: trimming empty <span>
callback.html:580:6: Warning: trimming empty <span>
readr-package.html:23:4: Warning: <img> attribute "align" not allowed for HTML5
readr-package.html:23:4: Warning: <img> attribute "align" not allowed for HTML5