1. Specify at Configuration - Editor - Word Wrap/Tab Settings the indent space value you want. You can specify different values based on the file extension. See also the help about this configuration page.
Now you can indent a SELECTION with the TAB key and unindent it with Shift+Tab or use the ReIndent Selection feature from menu Format.
Add/Remove Indents are for general usage for files without special indent setting or a file has indents but with the wrong number of indent spaces and you want to reformat it. They can be also used for just inserting or deleting preceding white-spaces.
Next turn on at Configuration - Editor - Display - Formatting the option Auto Indent
. The indent and unindent strings for this feature are specified in the wordfile for every language with a syntax highlighting definition, if you have to to modify it to your needs. See also help about Syntax Highlighting.
2. UltraEdit is a general text editor and not an editor designed for editing HTML files only. Although there are some features especially for editing HTML files like the multi-language syntax highlighting feature and the builtin code folding feature, a "go to the matching tag" feature is not available. Sure, the builtin code folding feature for HTML knows some block elements like <p></p>, <table></table>, etc., but all inline elements are for example not supported.
At Finding matching tag
and How to Select all chars between 2 html tags?
you can read about possible workarounds. They are not 100% perfect because they will fail in case of nested elements, but better than nothing.
UEStudio has more features for HTML editing, for example the Close Tag
feature which finds the nearest unclosed tag and closes it.